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What are Infrared Waves?

Infrared waves, or infrared light, are part of the electromagnetic spectrum. People encounter Infrared waves every day; the human eye cannot see it, but humans can detect it as heat.

A remote control uses light waves just beyond the visible spectrum of light-infrared light waves-to change channels on your TV. This region of the spectrum is divided into near-, mid-, and far-infrared. The region from 8 to 15 microns (µm) is referred to by Earth scientists as thermal infrared since these wavelengths are best for studying the longwave thermal energy radiating from our planet.

DISCOVERY OF INFRARED

In 1800, William Herschel conducted an experiment measuring the difference in temperature between the colors in the visible spectrum. He placed thermometers within each color of the visible spectrum. The results showed an increase in temperature from blue to red. When he noticed an even warmer temperature measurement just beyond the red end of the visible spectrum, Herschel had discovered infrared light!

THERMAL IMAGING

We can sense some infrared energy as heat. Some objects are so hot they also emit visible light-such as a fire does. Other objects, such as humans, are not as hot and only emit only infrared waves. Our eyes cannot see these infrared waves but instruments that can sense infrared energy-such as night-vision goggles or infrared cameras-allow us to "see" the infrared waves emitting from warm objects such as humans and animals. The temperatures for the images below are in degrees Fahrenheit.

COOL ASTRONOMY

Many objects in the universe are too cool and faint to be detected in visible light but can be detected in the infrared. Scientists are beginning to unlock the mysteries of cooler objects across the universe such as planets, cool stars, nebulae, and many more, by studying the infrared waves they emit.

The Cassini spacecraft captured this image of Saturn's aurora using infrared waves. The aurora is shown in blue, and the underlying clouds are shown in red. These aurorae are unique because they can cover the entire pole, whereas aurorae around Earth and Jupiter are typically confined by magnetic fields to rings surrounding the magnetic poles. The large and variable nature of these aurorae indicates that charged particles streaming in from the Sun are experiencing some type of magnetism above Saturn that was previously unexpected.

SEEING THROUGH DUST

Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes. The James Webb Space Telescope (JWST) has three infrared instruments to help study the origins of the universe and the formation of galaxies, stars, and planets.

A pillar composed of gas and dust in the Carina Nebula is illuminated by the glow from nearby massive stars shown below in the visible light image from the Hubble Space Telescope. Intense radiation and fast streams of charged particles from these stars are causing new stars to form within the pillar. Most of the new stars cannot be seen in the visible-light image (left) because dense gas clouds block their light. However, when the pillar is viewed using the infrared portion of the spectrum (right), it practically disappears, revealing the baby stars behind the column of gas and dust.

MONITORING THE EARTH

To astrophysicists studying the universe, infrared sources such as planets are relatively cool compared to the energy emitted from hot stars and other celestial objects. Earth scientists study infrared as the thermal emission (or heat) from our planet. As incident solar radiation hits Earth, some of this energy is absorbed by the atmosphere and the surface, thereby warming the planet. This heat is emitted from Earth in the form of infrared radiation. Instruments onboard Earth observing satellites can sense this emitted infrared radiation and use the resulting measurements to study changes in land and sea surface temperatures.

There are other sources of heat on the Earth's surface, such as lava flows and forest fires. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Aqua and Terra satellites uses infrared data to monitor smoke and pinpoint sources of forest fires. This information can be essential to firefighting efforts when fire reconnaissance planes are unable to fly through the thick smoke. Infrared data can also enable scientists to distinguish flaming fires from still-smoldering burn scars.

The global image on the right is an infrared image of the Earth taken by the GOES 6 satellite in 1986. A scientist used temperatures to determine which parts of the image were from clouds and which were land and sea. Based on these temperature differences, he colored each separately using 256 colors, giving the image a realistic appearance.

Why use the infrared to image the Earth? While it is easier to distinguish clouds from land in the visible range, there is more detail in the clouds in the infrared. This is great for studying cloud structure. For instance, note that darker clouds are warmer, while lighter clouds are cooler. Southeast of the Galapagos, just west of the coast of South America, there is a place where you can distinctly see multiple layers of clouds, with the warmer clouds at lower altitudes, closer to the ocean that's warming them.

We know, from looking at an infrared image of a cat, that many things emit infrared light. But many things also reflect infrared light, particularly near infrared light. Learn more about REFLECTED Near-infrared radiation.

Top of Page | Next: Reflected Near-Infrared Waves

Citation

APA

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Infrared Waves. Retrieved [insert date - e.g. August 10, 2016], from NASA Science website: http://science.nasa.gov/ems/07_infraredwaves

MLA

Science Mission Directorate. "Infrared Waves" NASA Science. 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov /ems/07_infraredwaves

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What are Infrared Waves? Infrared waves, or infrared light, are part of the electromagnetic spectrum. People encounter Infrared waves every day; the human eye cannot see it, but humans can detect it as heat. A remote control uses light waves just beyond the visible spectrum of light-infrared light waves-to change channels on your TV.

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Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, they are either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.

Specialized instruments onboard NASA spacecraft and airplanes collect data on how electromagnetic waves behave when they interact with matter. These data can reveal the physical and chemical composition of matter.

Reflection

Reflection is when incident light (incoming light) hits an object and bounces off. Very smooth surfaces such as mirrors reflect almost all incident light.

The color of an object is actually the wavelengths of the light reflected while all other wavelengths are absorbed. Color, in this case, refers to the different wavelengths of light in the visible light spectrum perceived by our eyes. The physical and chemical composition of matter determines which wavelength (or color) is reflected.

This reflective behavior of light is used by lasers onboard NASA's Lunar Reconnaissance Orbiter to map the surface of the Moon. The instrument measures the time it takes a laser pulse to hit the surface and return. The longer the response time, the farther away the surface and lower the elevation. A shorter response time means the surface is closer or higher in elevation. In this image of the Moon's southern hemisphere, low elevations are shown as purple and blue, and high elevations are shown in red and brown.

Absorption

Absorption occurs when photons from incident light hit atoms and molecules and cause them to vibrate. The more an object's molecules move and vibrate, the hotter it becomes. This heat is then emitted from the object as thermal energy.

Some objects, such as darker colored objects, absorb more incident light energy than others. For example, black pavement absorbs most visible and UV energy and reflects very little, while a light-colored concrete sidewalk reflects more energy than it absorbs. Thus, the black pavement is hotter than the sidewalk on a hot summer day. Photons bounce around during this absorption process and lose bits of energy to numerous molecules along the way. This thermal energy then radiates in the form of longer wavelength infrared energy.

Thermal radiation from the energy-absorbing asphalt and roofs in a city can raise its surface temperature by as much as 10° Celsius. The Landsat 7 satellite image below shows the city of Atlanta as an island of heat compared to the surrounding area. Sometimes this warming of air above cities can influence weather, which is called the "urban heat island" effect.

Diffraction

Diffraction is the bending and spreading of waves around an obstacle. It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength. An instrument called a spectrometer uses diffraction to separate light into a range of wavelengths-a spectrum. In the case of visible light, the separation of wavelengths through diffraction results in a rainbow.

A spectrometer uses diffraction (and the subsequent interference) of light from slits or gratings to separate wavelengths. Faint peaks of energy at specific wavelengths can then be detected and recorded. A graph of these data is called a spectral signature. Patterns in a spectral signature help scientists identify the physical condition and composition of stellar and interstellar matter.

The graph below from the SPIRE infrared spectrometer onboard the ESA (European Space Agency) Herschel space telescope reveals strong emission lines from carbon monoxide (CO), atomic carbon, and ionized nitrogen in Galaxy M82.

Scatter

Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object.

The sky appears blue because of this scattering behavior. Light at shorter wavelengths-blue and violet-is scattered by nitrogen and oxygen as it passes through the atmosphere. Longer wavelengths of light-red and yellow-transmit through the atmosphere. This scattering of light at shorter wavelengths illuminates the skies with light from the blue and violet end of the visible spectrum. Even though violet is scattered more than blue, the sky looks blue to us because our eyes are more sensitive to blue light.

Aerosols in the atmosphere can also scatter light. NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite can observe the scattering of laser pulses to "see" the distributions of aerosols from sources such as dust storms and forest fires. The image below shows a volcanic ash cloud drifting over Europe from an eruption of Iceland's Eyjafjallajökull Volcano in 2010.

Refraction

Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles.

Top of Page | Next: Visualization: From Energy to Image

Citation

APA

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Wave Behaviors. Retrieved [insert date - e.g. August 10, 2016], from NASA Science website: http://science.nasa.gov/ems/03_behaviors

MLA

Science Mission Directorate. "Wave Behaviors" NASA Science. 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/03_behaviors

# data[].attributes.attachment_link.description
Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, they are either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.

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What is the visible light spectrum?

The visible light spectrum is the segment of the electromagnetic spectrum that the human eye can view. More simply, this range of wavelengths is called visible light. Typically, the human eye can detect wavelengths from 380 to 700 nanometers.

WAVELENGTHS OF VISIBLE LIGHT

All electromagnetic radiation is light, but we can only see a small portion of this radiation-the portion we call visible light. Cone-shaped cells in our eyes act as receivers tuned to the wavelengths in this narrow band of the spectrum. Other portions of the spectrum have wavelengths too large or too small and energetic for the biological limitations of our perception.

As the full spectrum of visible light travels through a prism, the wavelengths separate into the colors of the rainbow because each color is a different wavelength. Violet has the shortest wavelength, at around 380 nanometers, and red has the longest wavelength, at around 700 nanometers.

THE SUN'S CORONA

The Sun is the dominant source for visible-light waves our eyes receive. The outer-most layer of the Sun's atmosphere, the corona, can be seen in visible light. But it is so faint it cannot not be seen except during a total solar eclipse because the bright photosphere overwhelms it. The photograph below was taken during a total eclipse of the Sun where the photosphere and chromosphere are almost completely blocked by the moon. The tapered patterns-coronal streamers-around the Sun are formed by the outward flow of plasma that is shaped by magnetic field lines extending millions of miles into space.

COLOR AND TEMPERATURE

As objects grow hotter, they radiate energy dominated by shorter wavelengths, changing color before our eyes. A flame on a blow torch shifts from reddish to bluish in color as it is adjusted to burn hotter. In the same way, the color of stars tells scientists about their temperature.

Our Sun produces more yellow light than any other color because its surface temperature is 5,500°C. If the Sun's surface were cooler-say 3,000°C-it would look reddish, like the star Betelgeuse. If the Sun were hotter-say, 12,000°C-it would look blue, like the star Rigel.

Isaac Newton's experiment in 1665 showed that a prism bends visible light and that each color refracts at a slightly different angle depending on the wavelength of the color.

SPECTRA AND SPECTRAL SIGNATURES

Close examination of the visible-light spectrum from our Sun and other stars reveals a pattern of dark lines-called absorption lines. These patterns can provide important scientific clues that reveal hidden properties of objects throughout the universe. Certain elements in the Sun's atmosphere absorb certain colors of light. These patterns of lines within spectra act like fingerprints for atoms and molecules. Looking at the Sun's spectrum, for example, the fingerprints for elements are clear to those knowledgeable about those patterns.

Patterns are also evident in a graph of an object's reflectance. Elements, molecules, and even cell structures have unique signatures of reflectance. A graph of an object's reflectance across a spectrum is called a spectral signature. Spectral signatures of different Earth features within the visible light spectrum ARE shown below.

ACTIVE REMOTE SENSING-ALTIMETRY

Laser altimetry is an example of active remote sensing using visible light. NASA's Geoscience Laser Altimeter System (GLAS) instrument onboard the Ice, Cloud, and land Elevation Satellite (ICESat) enabled scientists to calculate the elevation of Earth's polar ice sheets using lasers and ancillary data. Changes in elevation over time help to estimate variations in the amount of water stored as ice on our planet. The image below shows elevation data over the West Antarctic Ice Streams.

Laser altimeters can also make unique measurements of the heights and characteristics of clouds, as well as the top and structure of the vegetation canopy of forests. They can also sense the distribution of aerosols from sources such as dust storms and forest fires.

Top of Page | Next: Ultraviolet Waves

Citation

APA

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Visible Light. Retrieved [insert date - e.g. August 10, 2016], from NASA Science website: http://science.nasa.gov/ems/09_visiblelight

MLA

Science Mission Directorate. "Visible Light" NASA Science. 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/09_visiblelight

# data[].attributes.attachment_link.description
What is the visible light spectrum? The visible light spectrum is the segment of the electromagnetic spectrum that the human eye can view. More simply, this range of wavelengths is called visible light. Typically, the human eye can detect wavelengths from 380 to 700 nanometers.

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Alaska Science Forum (A great paper about how "hearing" an earthquake before it hits: the speed of sound versus the speed of seismic waves.)

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Longitudinal Wave

A wave is a disturbance of a medium which transports energy through the medium without permanently transporting matter. In a wave, particles of the medium are temporarily displaced and then return to their original position. There are a variety of ways to categorize waves. One way to categorize waves is to say that there are longitudinal and transverse waves. In a transverse wave, particles of the medium are displaced in a direction perpendicular to the direction of energy transport. In a longitudinal wave, particles of the medium are displaced in a direction parallel to energy transport. The animation below depicts a longitudinal pulse in a medium.

The animation portrays a medium as a series of particles connected by springs. As one individual particle is disturbed, it transmits the disturbance to the next interconnected particle. This disturbance continues to be passed on to the next particle. The result is that energy is transported from one end of the medium to the other end of the medium without the actual transport of matter. In this type of wave - a longitudinal wave - the particles of the medium vibrate in a direction parallel to the direction of energy transport.

For more information on physical descriptions of waves, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:

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A wave is a disturbance of a medium which transports energy through the medium without permanently transporting matter. In a wave, particles of the medium are temporarily displaced and then return to their original position. There are a variety of ways to categorize waves.

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In physics, a transverse wave is a moving wave whose oscillations are perpendicular to the direction of the wave.

A simple example is given by the waves that can be created on a horizontal length of string by anchoring one end and moving the other end up and down.

Another example is the waves that are created on the membrane of a drum. The waves propagate in directions that are parallel to the membrane plane, but the membrane itself gets displaced up and down, perpendicular to that plane.

Light is another example of a transverse wave, where the oscillations are the electric and magnetic fields, which point at right angles to the ideal light rays that describe the direction of propagation.

Transverse waves commonly occur in elastic solids; the oscillations in this case are the displacement of the solid particles away from their relaxed position, in directions perpendicular to the propagation of the wave. Since those displacements correspond to a local shear deformation of the material, a transverse wave of this nature is called a shear wave. In seismology, shear waves are also called secondary waves or S-waves.

Transverse waves are contrasted with longitudinal waves, where the oscillations occur in the direction of the wave. The standard example of a longitudinal wave is a sound wave or "pressure wave" in gases, liquids, or solids, whose oscillations cause compression and expansion of the material through which the wave is propagating. Pressure waves are called "primary waves", or "P-waves" in geophysics.

Mathematically, the simplest kind of transverse wave is a plane linearly polarized sinusoidal one. "Plane" here means that the direction of propagation is unchanging and the same over the whole medium; " linearly polarized" means that the direction of displacement too is unchanging and the same over the whole medium; and the magnitude of the displacement is a sinusoidal function only of time and of position along the direction of propagation.

The motion of such a wave can be expressed mathematically as follows. Let d be the direction of propagation (a vector with unit length), and o any reference point in the medium. Let u be the direction of the oscillations (another unit-length vector perpendicular to d). The displacement of a particle at any point p of the medium and any time t (seconds) will be

where A is the wave's amplitude or strength, T is its period, v is the speed of propagation, and φ is its phase at o. All these parameters are real numbers. The symbol "*" denotes the inner product of two vectors.

By this equation, the wave travels in the direction d and the oscillations occur back and forth along the direction u. The wave is said to be linearly polarized in the direction u.

An observer that looks at a fixed point p will see the particle there move in a simple harmonic (sinusoidal) motion with period T seconds, with maximum particle displacement A in each sense; that is, with a frequency of f = 1/ T full oscillation cycles every second. A snapshot of all particles at a fixed time t will show the same displacement for all particles on each plane perpendicular to d, with the displacements in successive planes forming a sinusoidal pattern, with each full cycle extending along d by the wavelength λ = v T = v/ f. The whole pattern moves in the direction d with speed V.

The same equation describes a plane linearly polarized sinusoidal light wave, except that the "displacement" S( p, t) is the electric field at point p and time t. (The magnetic field will be described by the same equation, but with a "displacement" direction that is perpendicular to both d and u, and a different amplitude.)

Superposition principle

In a homogeneous elastic medium, complex oscillations (vibrations in a material or light flows) can be described as the superposition of many simple sinusoidal waves, either transverse (linearly polarized) or longitudinal.

The vibrations of a violin string, for example, can be analyzed as the sum of many transverse waves of different frequencies, that displace the string either up or down or left to right. The ripples in a pond can be analyzed as a combination of transverse and longitudinal waves ( gravity waves) that propagate together.

Circular polarization

If the medium is linear and allows multiple independent displacement directions for the same travel direction d, we can choose two mutually perpendicular directions of polarization, and express any wave linearly polarized in any other direction as a linear combination (mixing) of those two waves.

By combining two waves with same frequency, velocity, and direction of travel, but with different phases and independent displacement directions, one obtains a circularly or elliptically polarized wave. In such a wave the particles describe circular or ellitcal trajectories, instead of moving back and forth.

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In physics, a transverse wave is a moving wave whose oscillations are perpendicular to the direction of the wave. A simple example is given by the waves that can be created on a horizontal length of string by anchoring one end and moving the other end up and down.

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A mechanical wave is the type of wave that needs a medium to be transmitted, as waves of this type do not travel through a vacuum. The quiz below is designed to test your basic understanding on mechanical waves. It will take less than a minute and is a true or false question.

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In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result of the wind blowing over the water's surface. The contact distance in the direction of the wind is known as the . Waves in the oceans can travel thousands of kilometers before reaching land. Wind waves on Earth range in size from small ripples to waves over 30 m (100 ft) high, being limited by wind speed, duration, fetch, and water depth.

When directly generated and affected by local wind, a wind wave system is called a wind sea. Wind waves will travel in a great circle route after being generated - curving slightly left in the southern hemisphere and slightly right in the northern hemisphere. After moving out of the area of fetch, wind waves are called and can travel thousands of kilometers. A noteworthy example of this is waves generated south of Tasmania during heavy winds that will travel across the Pacific to southern California, producing desirable surfing conditions. Swell consists of wind-generated waves that are not significantly affected by the local wind at that time. They have been generated elsewhere and sometimes previously. Wind waves in the ocean are also called ocean surface waves and are mainly gravity waves, where gravity is the main equilibrium force.

Wind waves have a certain amount of randomness: subsequent waves differ in height, duration, and shape with limited predictability. They can be described as a stochastic process, in combination with the physics governing their generation, growth, propagation, and decay - as well as governing the interdependence between flow quantities such as the water surface movements, flow velocities, and water pressure. The key statistics of wind waves (both seas and swells) in evolving sea states can be predicted with wind wave models.

Although waves are usually considered in the water seas of Earth, the hydrocarbon seas of Titan may also have wind-driven waves.

The great majority of large breakers seen at a beach result from distant winds. Five factors influence the formation of the flow structures in wind waves:

1. Wind speed or strength relative to wave speed - the wind must be moving faster than the wave crest for energy transfer

2. The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)

3. Width of the area affected by fetch (at a right angle to the distance)

4. Wind duration - the time for which the wind has blown over the water.

5. Water depth

All of these factors work together to determine the size of the water waves and the structure of the flow within them.

The main dimensions associated with wave propagation are:

A fully developed sea has the maximum wave size theoretically possible for a wind of specific strength, duration, and fetch. Further exposure to that specific wind could only cause a dissipation of energy due to the breaking of wave tops and formation of "whitecaps". Waves in a given area typically have a range of heights. For weather reporting and for scientific analysis of wind wave statistics, their characteristic height over a period of time is usually expressed as significant wave height. This figure represents an average height of the highest one-third of the waves in a given time period (usually chosen somewhere in the range from 20 minutes to twelve hours), or in a specific wave or storm system. The significant wave height is also the value a "trained observer" (e.g. from a ship's crew) would estimate from visual observation of a sea state. Given the variability of wave height, the largest individual waves are likely to be somewhat less than twice the reported significant wave height for a particular day or storm.

Wave formation on an initially flat water surface by wind is started by a random distribution of normal pressure of turbulent wind flow over the water. This pressure fluctuation produces normal and tangential stresses in the surface water, which generates waves. It is assumed that:

1. The water is originally at rest.

2. The water is not viscous.

3. The water is irrotational.

4. There is a random distribution of normal pressure to the water surface from the turbulent wind.

5. Correlations between air and water motions are neglected.

The second mechanism involves wind shear forces on the water surface. John W. Miles suggested a surface wave generation mechanism that is initiated by turbulent wind shear flows based on the inviscid Orr-Sommerfeld equation in 1957. He found the energy transfer from the wind to the water surface is proportional to the curvature of the velocity profile of the wind at the point where the mean wind speed is equal to the wave speed. Since the wind speed profile is logarithmic to the water surface, the curvature has a negative sign at this point. This relation shows the wind flow transferring its kinetic energy to the water surface at their interface.

Assumptions:

1. two-dimensional parallel shear flow

2. incompressible, inviscid water and wind

3. irrotational water

4. slope of the displacement of the water surface is small

Generally, these wave formation mechanisms occur together on the water surface and eventually produce fully developed waves.

For example, if we assume a flat sea surface (Beaufort state 0), and a sudden wind flow blows steadily across the sea surface, the physical wave generation process follows the sequence:

1. Turbulent wind forms random pressure fluctuations at the sea surface. Ripples with wavelengths in the order of a few centimeters are generated by the pressure fluctuations. (The Phillips mechanism)

2. The winds keep acting on the initially rippled sea surface causing the waves to become larger. As the waves grow, the pressure differences get larger causing the growth rate to increase. Finally, the shear instability expedites the wave growth exponentially. (The Miles mechanism)

3. The interactions between the waves on the surface generate longer waves and the interaction will transfer wave energy from the shorter waves generated by the Miles mechanism to the waves which have slightly lower frequencies than the frequency at the peak wave magnitudes, then finally the waves will be faster than the crosswind speed (Pierson & Moskowitz).

Conditions necessary for a fully developed sea at given wind speeds, and the parameters of the resulting waves

Wind conditions

Wave size

Wind speed in one direction

Fetch

Wind duration

Average height

Average wavelength

Average period and speed

NOTE: Most of the wave speeds calculated from the wave length divided by the period are proportional to the square root of the wave length. Thus, except for the shortest wave length, the waves follow the deep water theory. The 28 ft long wave must be either in shallow water or intermediate depth.

Three different types of wind waves develop over time:

Capillary waves, or ripples, dominated by surface tension effects.

Gravity waves, dominated by gravitational and inertial forces.
Seas, raised locally by the wind.

Swells, which have traveled away from where they were raised by the wind, and have to a greater or lesser extent dispersed.

Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops. The restoring force that allows them to propagate is surface tension. Sea waves are larger-scale, often irregular motions that form under sustained winds. These waves tend to last much longer, even after the wind has died, and the restoring force that allows them to propagate is gravity. As waves propagate away from their area of origin, they naturally separate into groups of common direction and wavelength. The sets of waves formed in this manner are known as swells. The Pacific Ocean is 19,800km from Indonesia to the coast of Colombia and, based on an average wavelength of 76.5m, would have ~258,824 swells over that width.

Individual " rogue waves" (also called "freak waves", "monster waves", "killer waves", and "king waves") much higher than the other waves in the sea state can occur. In the case of the Draupner wave, its 25 m (82 ft) height was 2.2 times the significant wave height. Such waves are distinct from tides, caused by the Moon and Sun's gravitational pull, tsunamis that are caused by underwater earthquakes or landslides, and waves generated by underwater explosions or the fall of meteorites-all having far longer wavelengths than wind waves.

The largest ever recorded wind waves are not rogue waves, but standard waves in extreme sea states. For example, 29.1 m (95 ft) high waves were recorded on the RRS Discovery in a sea with 18.5 m (61 ft) significant wave height, so the highest wave was only 1.6 times the significant wave height. The biggest recorded by a buoy (as of 2011) was 32.3 m (106 ft) high during the 2007 typhoon Krosa near Taiwan.

Ocean waves can be classified based on: the disturbing force that creates them; the extent to which the disturbing force continues to influence them after formation; the extent to which the restoring force weakens or flattens them; and their wavelength or period. Seismic sea waves have a period of about 20 minutes, and speeds of 760 km/h (470 mph). Wind waves (deep-water waves) have a period of about 20 seconds.

Wave type

Typical wavelength

Disturbing force

Restoring force

Wind wave

60-150 m (200-490 ft)

Wind over ocean

Gravity

Large, variable; a function of basin size

Change in atmospheric pressure, storm surge

Gravity

Seismic sea wave (tsunami)

200 km (120 mi)

Faulting of sea floor, volcanic eruption, landslide

Gravity

Tide

Half the circumference of Earth

Gravitational attraction, rotation of Earth

Gravity

The speed of all ocean waves is controlled by gravity, wavelength, and water depth. Most characteristics of ocean waves depend on the relationship between their wavelength and water depth. Wavelength determines the size of the orbits of water molecules within a wave, but water depth determines the shape of the orbits. The paths of water molecules in a wind wave are circular only when the wave is traveling in deep water. A wave cannot "feel" the bottom when it moves through water deeper than half its wavelength because too little wave energy is contained in the small circles below that depth. Waves moving through water deeper than half their wavelength are known as deep-water waves. On the other hand, the orbits of water molecules in waves moving through shallow water are flattened by the proximity of the sea surface bottom. Waves in water shallower than 1/20 their original wavelength are known as shallow-water waves. Transitional waves travel through water deeper than 1/20 their original wavelength but shallower than half their original wavelength.

In general, the longer the wavelength, the faster the wave energy will move through the water. The relationship between the wavelength, period and velocity of any wave is:

where C is speed (celerity), L is the wavelength, and T is time, or period (in seconds). Thus the speed of the wave derives from the functional dependence of the wavelength on the period (the dispersion relation).

The speed of a deep-water wave may also be approximated by:

where g is the acceleration due to gravity, 9.8 meters (32 feet) per second squared. Because g and π (3.14) are constants, the equation can be reduced to:

when C is measured in meters per second and L in meters. Note that in both formulas the wave speed is proportional to the square root of the wavelength.

The speed of shallow-water waves is described by a different equation that may be written as:

where C is speed (in meters per second), g is the acceleration due to gravity, and d is the depth of the water (in meters). The period of a wave remains unchanged regardless of the depth of water through which it is moving. As deep-water waves enter the shallows and feel the bottom, however, their speed is reduced, and their crests "bunch up," so their wavelength shortens.

Spectral models

Sea state can be described by the sea wave spectrum or just wave spectrum . It is composed of a wave height spectrum (WHS) and a wave direction spectrum (WDS) . Many interesting properties about the sea state can be found from the wave spectra.

WHS describes the spectral density of wave height variance ("power") versus wave frequency, with dimension . The relationship between the spectrum and the wave amplitude for a wave component is:

[citation needed ]

Some WHS models are listed below.

where

(The latter model has since its creation improved based on the work of Phillips and Kitaigorodskii to better model the wave height spectrum for high wavenumbers.[20])

As for WDS, an example model of might be:

Thus the sea state is fully determined and can be recreated by the following function where is the wave elevation, is uniformly distributed between 0 and , and is randomly drawn from the directional distribution function [21]

As waves travel from deep to shallow water, their shape changes (wave height increases, speed decreases, and length decreases as wave orbits become asymmetrical). This process is called shoaling.

Wave refraction is the process that occurs when waves interact with the sea bed to slow the velocity of propagation as a function of wavelength and period. As the waves slow down in shoaling water, the crests tend to realign at a decreasing angle to the depth contours. Varying depths along a wave crest cause the crest to travel at different phase speeds, with those parts of the wave in deeper water moving faster than those in shallow water. This process continues while the depth decreases, and reverses if it increases again, but the wave leaving the shoal area may have changed direction considerably. Rays-lines normal to wave crests between which a fixed amount of energy flux is contained-converge on local shallows and shoals. Therefore, the wave energy between rays is concentrated as they converge, with a resulting increase in wave height.

Because these effects are related to a spatial variation in the phase speed, and because the phase speed also changes with the ambient current - due to the Doppler shift - the same effects of refraction and altering wave height also occur due to current variations. In the case of meeting an adverse current the wave steepens, i.e. its wave height increases while the wavelength decreases, similar to the shoaling when the water depth decreases.

Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave, is too great, breaking is inevitable.

Individual waves in deep water break when the wave steepness-the ratio of the wave height H to the wavelength λ-exceeds about 0.17, so for H > 0.17 λ. In shallow water, with the water depth small compared to the wavelength, the individual waves break when their wave height H is larger than 0.8 times the water depth h, that is H > 0.8 h. Waves can also break if the wind grows strong enough to blow the crest off the base of the wave.

In shallow water, the base of the wave is decelerated by drag on the seabed. As a result, the upper parts will propagate at a higher velocity than the base and the leading face of the crest will become steeper and the trailing face flatter. This may be exaggerated to the extent that the leading face forms a barrel profile, with the crest falling forward and down as it extends over the air ahead of the wave.

Three main types of breaking waves are identified by surfers or surf lifesavers. Their varying characteristics make them more or less suitable for surfing and present different dangers.

1. Spilling, or rolling: these are the safest waves on which to surf. They can be found in most areas with relatively flat shorelines. They are the most common type of shorebreak. The deceleration of the wave base is gradual, and the velocity of the upper parts does not differ much with height. Breaking occurs mainly when the steepness ratio exceeds the stability limit.

2. Plunging, or dumping: these break suddenly and can "dump" swimmers-pushing them to the bottom with great force. These are the preferred waves for experienced surfers. Strong offshore winds and long wave periods can cause dumpers. They are often found where there is a sudden rise in the seafloor, such as a reef or sandbar. Deceleration of the wave base is sufficient to cause upward acceleration and a significant forward velocity excess of the upper part of the crest. The peak rises and overtakes the forward face, forming a "barrel" or "tube" as it collapses.

3. Surging: these may never actually break as they approach the water's edge, as the water below them is very deep. They tend to form on steep shorelines. These waves can knock swimmers over and drag them back into deeper water.

When the shoreline is near vertical, waves do not break but are reflected. Most of the energy is retained in the wave as it returns to seaward. Interference patterns are caused by superposition of the incident and reflected waves, and the superposition may cause localized instability when peaks cross, and these peaks may break due to instability. (see also clapotic waves)

Wind waves are mechanical waves that propagate along the interface between water and air; the restoring force is provided by gravity, and so they are often referred to as surface gravity waves. As the wind blows, pressure and friction perturb the equilibrium of the water surface and transfer energy from the air to the water, forming waves. The initial formation of waves by the wind is described in the theory of Phillips from 1957, and the subsequent growth of the small waves has been modeled by Miles, also in 1957.

In linear plane waves of one wavelength in deep water, parcels near the surface move not plainly up and down but in circular orbits: forward above and backward below (compared to the wave propagation direction). As a result, the surface of the water forms not an exact sine wave, but more a trochoid with the sharper curves upwards-as modeled in trochoidal wave theory. Wind waves are thus a combination of transversal and longitudinal waves.

When waves propagate in shallow water, (where the depth is less than half the wavelength) the particle trajectories are compressed into ellipses.

In reality, for finite values of the wave amplitude (height), the particle paths do not form closed orbits; rather, after the passage of each crest, particles are displaced slightly from their previous positions, a phenomenon known as Stokes drift.

As the depth below the free surface increases, the radius of the circular motion decreases. At a depth equal to half the wavelength λ, the orbital movement has decayed to less than 5% of its value at the surface. The phase speed (also called the celerity) of a surface gravity wave is - for pure periodic wave motion of small- amplitude waves - well approximated by

where

In deep water, where , so and the hyperbolic tangent approaches , the speed approximates

In SI units, with in m/s, , when is measured in metres. This expression tells us that waves of different wavelengths travel at different speeds. The fastest waves in a storm are the ones with the longest wavelength. As a result, after a storm, the first waves to arrive on the coast are the long-wavelength swells.

For intermediate and shallow water, the Boussinesq equations are applicable, combining frequency dispersion and nonlinear effects. And in very shallow water, the shallow water equations can be used.

If the wavelength is very long compared to the water depth, the phase speed (by taking the limit of c when the wavelength approaches infinity) can be approximated by

On the other hand, for very short wavelengths, surface tension plays an important role and the phase speed of these gravity-capillary waves can (in deep water) be approximated by

where

S = surface tension of the air-water interface;

= density of the water.[32]

When several wave trains are present, as is always the case in nature, the waves form groups. In deep water, the groups travel at a group velocity which is half of the phase speed. Following a single wave in a group one can see the wave appearing at the back of the group, growing, and finally disappearing at the front of the group.

As the water depth decreases towards the coast, this will have an effect: wave height changes due to wave shoaling and refraction. As the wave height increases, the wave may become unstable when the crest of the wave moves faster than the trough. This causes surf, a breaking of the waves.

The movement of wind waves can be captured by wave energy devices. The energy density (per unit area) of regular sinusoidal waves depends on the water density , gravity acceleration and the wave height (which, for regular waves, is equal to twice the amplitude, ):

The velocity of propagation of this energy is the group velocity.

Surfers are very interested in the wave forecasts. There are many websites that provide predictions of the surf quality for the upcoming days and weeks. Wind wave models are driven by more general weather models that predict the winds and pressures over the oceans, seas, and lakes.

Wind wave models are also an important part of examining the impact of shore protection and beach nourishment proposals. For many beach areas there is only patchy information about the wave climate, therefore estimating the effect of wind waves is important for managing littoral environments.

A wind-generated wave can be predicted based on two parameters: wind speed at 10 m above sea level and wind duration, which must blow over long periods of time to be considered fully developed. The significant wave height and peak frequency can then be predicted for a certain fetch length.

Ocean water waves generate land seismic waves that propagate hundreds of kilometers into the land. These seismic signals usually have a period of 6 ± 2 seconds. Such recordings were first reported and understood in about 1900.

There are two types of seismic "ocean waves". The primary waves are generated in shallow waters by direct water wave-land interaction and have the same period as the water waves (10 to 16 seconds). The more powerful secondary waves are generated by the superposition of ocean waves of equal period traveling in opposite directions, thus generating standing gravity waves - with an associated pressure oscillation at half the period, which is not diminishing with depth. The theory for microseism generation by standing waves was provided by Michael Longuet-Higgins in 1950 after in 1941 Pierre Bernard suggested this relation with standing waves on the basis of observations.

Airy wave theory - Fluid dynamics theory on the propagation of gravity waves

Breakwater (structure) - Structure constructed on coasts as part of coastal management or to protect an anchorage

Boussinesq approximation (water waves) - Approximation valid for weakly non-linear and fairly long waves

Clapotis - Non-breaking standing wave pattern

Cross sea - Sea state with two wave systems traveling at oblique angles

Gravity wave - Wave in or at the interface between fluids where gravity is the main equilibrium force

Internal wave - Type of gravity waves that oscillate within a fluid medium

Luke's variational principle - Mathematics of surface waves on a fluid

Mild-slope equation - Physics phenomenon and formula

Rogue wave - Unexpectedly large transient ocean surface wave

Shallow water equations - Set of partial differential equations that describe the flow below a pressure surface in a fluid

Tsunami - Series of water waves caused by the displacement of a large volume of a body of water

Wave power - Transport of energy by wind waves, and the capture of that energy to do useful work

Wave radar - Technology for measuring surface waves on water

Wave setup - The increase in mean water level due to the presence of breaking waves

Waves and shallow water - Effect of shallow water on a surface gravity wave

1. ^ Tolman, H. L. (23 June 2010). Mahmood, M.F. (ed.). CBMS Conference Proceedings on Water Waves: Theory and Experiment (PDF). Howard University, US, 13-18 May 2008: World Scientific Publications. ISBN 978-981-4304-23-8.

2. ^ Lorenz, R. D.; Hayes, A. G. (2012). "The Growth of Wind-Waves in Titan's Hydrocarbon Seas". Icarus. 219 (1): 468-475. Bibcode:2012Icar..219..468L. doi:10.1016/j.icarus.2012.03.002.

3. ^ Heslar, Michael F.; Barnes, Jason W.; Soderblom, Jason M.; Seignovert, Benoît; Dhingra, Rajani D.; Sotin, Christophe (2020-08-14). "Tidal Currents Detected in Kraken Mare Straits from Cassini VIMS Sun Glitter Observations". The Planetary Science Journal. 1 (2): 35. arXiv:. Bibcode:2020PSJ.....1...35H. doi:10.3847/PSJ/aba191. ISSN 2632-3338. S2CID 220301577.

4. ^ Hasselmann, K.; et al. (1973). "Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP)". Ergnzungsheft zur Deutschen Hydrographischen Zeitschrift Reihe A. 8 (12): 95. hdl:10013/epic.20654.

5. ^ Pierson, Willard J.; Moskowitz, Lionel (15 December 1964). "A proposed spectral form for fully developed wind seas based on the similarity theory of S. A. Kitaigorodskii". Journal of Geophysical Research. 69 (24): 5181-5190. Bibcode:1964JGR....69.5181P. doi:10.1029/JZ069i024p05181.

6. ^ Holliday, Naomi P.; Yelland, Margaret J.; Pascal, Robin; Swail, Val R.; Taylor, Peter K.; Griffiths, Colin R.; Kent, Elizabeth (2006). "Were extreme waves in the Rockall Trough the largest ever recorded?". Geophysical Research Letters. 33 (L05613). Bibcode:2006GeoRL..3305613H. doi:10.1029/2005GL025238.

7. ^ P. C. Liu; H. S. Chen; D.-J. Doong; C. C. Kao; Y.-J. G. Hsu (11 June 2008). "Monstrous ocean waves during typhoon Krosa". Annales Geophysicae. 26 (6): 1327-1329. Bibcode:2008AnGeo..26.1327L. doi:10.5194/angeo-26-1327-2008.

8. ^ Munk, Walter H. (1950). "Proceedings 1st International Conference on Coastal Engineering". Long Beach, California: ASCE: 1-4.

9. ^ International Ship and Offshore Structures Congress

10. ^ Pierson, W. J.; Moscowitz, L. (1964), "A proposed spectral form for fully developed wind seas based on the similarity theory of S A Kitaigorodskii", Journal of Geophysical Research, 69 (24): 5181-5190, Bibcode:1964JGR....69.5181P, doi:10.1029/JZ069i024p05181

11. ^ Elfouhaily, T.; Chapron, B.; Katsaros, K.; Vandemark, D. (July 15, 1997). "A unified directional spectrum for long and short wind-driven waves" (PDF). Journal of Geophysical Research. 102 (C7): 15781-15796. Bibcode:1997JGR...10215781E. doi:10.1029/97jc00467.

12. ^ Jefferys, E. R. (1987), "Directional seas should be ergodic", Applied Ocean Research, 9 (4): 186-191, doi:10.1016/0141-1187(87)90001-0

13. ^ Longuet-Higgins, M. S.; Stewart, R. W. (1964). "Radiation stresses in water waves; a physical discussion, with applications". Deep-Sea Research. 11 (4): 529-562. Bibcode:1964DSRA...11..529L. doi:10.1016/0011-7471(64)90001-4.

14. ^ Gulrez, Tauseef; Hassanien, Aboul Ella (2011-11-13). Advances in Robotics and Virtual Reality. Springer Science & Business Media. ISBN 9783642233630.

15. ^ Phillips, O. M. (1957). "On the generation of waves by turbulent wind". Journal of Fluid Mechanics. 2 (5): 417-445. Bibcode:1957JFM.....2..417P. doi:10.1017/S0022112057000233. S2CID 116675962.

16. Figure 6 from: ^ Wiegel, R. L.; Johnson, J. W. (1950). "Proceedings 1st International Conference on Coastal Engineering". Long Beach, California: ASCE: 5-21.

17. For the particle trajectories within the framework of linear wave theory, see for instance: ^
Phillips (1977), page 44.
Lamb, H. (1994). Hydrodynamics (6th ed.). Cambridge University Press. ISBN 978-0-521-45868-9. Originally published in 1879, the 6th extended edition appeared first in 1932. See §229, page 367.
See page 33.

18. A good illustration of the wave motion according to linear theory is given by ^ Prof. Robert Dalrymple's Java applet Archived 2017-11-14 at the Wayback Machine.

19. For nonlinear waves, the particle paths are not closed, as found by ^ George Gabriel Stokes in 1847, see the original paper by Stokes. Or in Phillips (1977), page 44: "To this order, it is evident that the particle paths are not exactly closed ... pointed out by Stokes (1847) in his classical investigation".

20. Solutions of the particle trajectories in fully nonlinear periodic waves and the Lagrangian wave period they experience can for instance be found in:^
J. M. Williams (1981). "Limiting gravity waves in water of finite depth". Philosophical Transactions of the Royal Society A. 302 (1466): 139-188. Bibcode:1981RSPTA.302..139W. doi:10.1098/rsta.1981.0159. S2CID 122673867.

21. In deep water, the ^ group velocity is half the phase velocity, as is shown here. Another reference is [1].

22. ^ Wood, AMM & Fleming, CA 1981, Coastal hydraulics, John Wiley & Sons, New York

23. ^ Bernard, P. (1941). "Sur certaines proprietes de la boule etudiees a l'aide des enregistrements seismographiques". Bulletin de l'Institut Océanographique de Monaco. 800: 1-19.

Scientific

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In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result of the wind blowing over the water's surface. The contact distance in the direction of the wind is known as the .

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by Ron Kurtus (revised 1 December 2009)

Sound waves can be classified into three groups, according to their frequency ranges. Infrasound consists of frequencies below 20 Hz, audible sound consists of frequencies between 20 Hz and 20,000 Hz (20 kilohertz), and ultrasound consists of frequencies over 20 kHz.

Each type of sound is a series of longitudinal or compression waves that move through air or other materials. Sound does not travel in a vacuum.

Like any waveform, sound has the characteristics of wavelength, frequency, amplitude, and speed or velocity. Sound waves are created by the vibration of some object, like the cone in a radio loudspeaker. The waves are detected when they cause a detector to vibrate.

Questions you may have include:

What is a description of sound?

What are the characteristics of sound waves?

How is sound created and detected?

This lesson will answer those questions. Useful tool: Units Conversion

Sound is a mechanical vibration

Sound is a regular mechanical vibration that travels through matter as a waveform. It consists of longitudinal or compression waves in matter.

Travels through matter

Although it is commonly associated in air, sound will readily travel through many materials, such as water and steel. Some insulating materials absorb much of the sound waves, preventing the waves from penetrating the material.

Does not travel in vacuum

Because sound is the vibration of matter, it does not travel through a vacuum or in outer space.

When you see movies or TV shows about battles in outer space, you should only be able to see an explosion but not hear it. The sounds are added for dramatic effect.

However, note that in outer space, there are actually some widely-spaced atoms and molecules floating around. But since they are so far apart, regular wave motion would not be great enough to detect.

Sound waves are different than light waves

Light and radio waves are electromagnetic waves. They are completely different than sound, which is vibration of matter. Electromagnetic waves are related to electrical and magnetic fields and readily travel through space.

Sound is a longitudinal wave

The rapid back-and-forth vibration of an object creates the longitudinal or compression waves of sound. Longitudinal waves are waves that oscillate in the same path that the sound wave is moving. This is different than the up and down or transverse motion of a water wave.

(See General Wave Motion for more information.)

Transverse wave (water wave)

Longitudinal or compression wave (sound)

The illustrations above show a comparison of a transverse wave-such as a water wave-and the compression wave of sound.

Characteristics of sound

A sound wave has the same characteristics as any other type of waveform. It has wavelength, frequency, velocity and amplitude.

Wavelength

Wavelength is the distance from one crest to another of a wave. Since sound is a compression wave, the wavelength is the distance between maximum compressions.

Speed or velocity

The sound waveform moves at approximately 344 meters/second, 1130 feet/sec. or 770 miles per hour at room temperature of 20 oC (70 o F).

( See Speed of Sound in a Gas for more information.)

Frequency

The frequency of sound is the rate at which the waves pass a given point. It is also the rate at which a guitar string or a loudspeaker vibrates.

The relationship between velocity, wavelength and frequency is:

velocity = wavelength x frequency

Amplitude

Since sound is a compression wave, its amplitude corresponds to how much the wave is compressed, as compared to areas of little compression. Thus, it is sometimes called pressure amplitude.

Creating and detecting sounds

Creating and detecting sounds are similar effects, but opposite. They demonstrate the duality of nature.

Creating sound

Whenever an object in air vibrates, it causes longitudinal or compression waves in the air. These waves move away from the object as sound. There are many forms of the vibration, some not so obvious.

The back and forth movement of a loudspeaker cone, guitar string or drum head result in compression waves of sound. When you speak, your vocal cords also vibrate, creating sound.

Blowing across a bottle top can also create sound. In this case, the air inside the bottle goes in a circular motion, resulting in sound waves being formed. Wind blowing through trees can also create sound this indirect way.

Sound can also be created by vibrating an object in a liquid such as water or in a solid such as iron. A train rolling on a steel railroad track will create a sound wave that travels through the tracks. They will then vibrate, creating sound in air that you can hear, while the train may be a great distance away.

Detecting sound

When a sound wave strikes an object, it can cause the object to vibrate. This leads to the method to detect sound, which requires changing that vibration into some other type of signal-usually electrical.

The main way you detect or sense sounds is through your ears. The sound waves vibrate your ear drum, which goes to the inner ear and is changed to nerve signals you can sense.

You can also feel sounds. Stand in front of a stereo or hi-fi loudspeaker on at full volume, and you can feel some of the vibrations from the music.

There are mechanical devices that detect sounds, such as the microphone. The sound vibrates a membrane, which creates an electric signal that is amplified and recorded.

( Also see Detecting Sounds for more information.)

Summary

Sound consists of longitudinal or compression waves that move through air or other materials. It does not travel in a vacuum. Sound has the characteristics of wavelength, frequency, speed and amplitude. Sound waves are created by the vibration of some object and are detected when they cause a detector to vibrate.

Sound out problems to solve them

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by Ron Kurtus (revised 1 December 2009) Sound waves can be classified into three groups, according to their frequency ranges. Infrasound consists of frequencies below 20 Hz, audible sound consists of frequencies between 20 Hz and 20,000 Hz (20 kilohertz), and ultrasound consists of frequencies over 20 kHz.

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What It Means

Pandemonium refers to a situation in which a crowd or mass of people act in a wild, uncontrolled, or violent way because they are afraid, excited, or confused.

// Pandemonium ensued when a power failure knocked out the city's traffic lights during rush hour.
See the entry >
pandemonium in Context

I went to the ______ store to buy a birthday card.

Did You Know?

When John Milton needed a name for the gathering place of all demons for Paradise Lost, he turned to the classics as any sensible 17th-century writer would. Pandæmonium, as the capital of Hell is known in the epic poem, combines the Greek prefix pan-, meaning "all," with the Late Latin daemonium, meaning "evil spirit." ( Daemonium itself traces back to the far more innocuous Greek word daímōn, meaning "spirit" or "divine power.") Over time, Pandæmonium (or Pandemonium) came to designate all of hell and was used as well for earthbound dens of wickedness and sin. By the late-18th century, the word implied a place or state of confusion or uproar, and from there, it didn't take long for pandemonium to become associated with states of utter disorder and wildness.

Test Your Vocabulary

Fill in the blanks to complete a word meaning "hell" or "eternal damnation": p _ r _ i _ _ o n.

VIEW THE ANSWER

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When John Milton needed a name for the gathering place of all demons for Paradise Lost, he turned to the classics as any sensible 17th-century writer would. Pandæmonium, as the capital of Hell is

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Convection currents in a lava lamp - IGCSE Physics

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