# data[].attributes.attachment_link.title
Why Your Phone Has To Be On Airplane Mode During A Flight

# data[].attributes.attachment_link.title
Introduction to the Electromagnetic Spectrum - NASA Science

# data[].attributes.attachment_link.description
What is Electromagnetic energy? Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and a…

# data[].attributes.attachment_link.readable_text
This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they'd like an expert to answer. All questions are welcome - serious, weird or wacky!

Hi. My name is Abhilasa, I am 9 years old and I am from Melbourne and my question is: What happens in our bodies when we sleep? Thank you. - Abhilasa, 9, Melbourne.

Most of us might think sleep is like a light switch. That is, we go to bed, shut our eyes, and our body "switches off". Then, when we open our eyes and wake up in the morning, we are "switching on" for the day.

But that is not true. There are changes that happen in our body during the night when we sleep.

Read more: Curious Kids: Why do our brains freak us out with scary dreams?

The most interesting part of our body that changes during sleep is our brain. We know this because people have been measuring how active our brain is during sleep since the 1950s. They have done this by gluing small circular pieces of metal attached to wires (known as electrodes) to our head and near our eyes. These electrodes have shown that when we sleep, our bodies do not "switch off". In fact, there are many things that our brain and eyes are doing.

Electrodes measure the waves produced by our brains. When we are awake, there are lots of these waves, but they are really small. So lots of tiny waves mean there is a lot of activity in our brains. Also, when we are awake, our eyes are moving around and looking at things - left and right, up and down, and all over the place.

When we shut our eyes to fall asleep, we begin to relax and fall into a very light sleep, which has been called "stage one" sleep. Our eyes begin to move left and right, back and forth, very slowly, many times, and very smoothly. It's called "slow rolling eye movements" - and it's something we cannot do when we are awake. During stage one sleep, our brains start to produce slightly bigger waves, and fewer of them.

Next is "stage two sleep", which is a slightly deeper sleep than stage one sleep. There's not much that is special about what our eyes are doing, but there's a special wave that our brain makes called a "sleep spindle". It's a bit like the brainwaves seen in stage one sleep, but short, sharp and shiny.

Read more: Curious Kids: Do animals sleep like people? Do snails sleep in their shells?

The next stages of sleep are deep ones, and they are called stage three and four sleep. These stages are really deep, and really hard to wake up from. The brainwaves become really big - like those ones you might have seen at the beach.

Finally, one of the most interesting stages of sleep is called "REM sleep". REM stands for Rapid Eye Movement - so it means our eyes are moving all over the place during sleep - like when we're awake. And our brainwaves are very similar to when we're awake. But it's during REM sleep that we dream.

Our brains move through the different stages of sleep - from one, two, three, four and then back to REM sleep. We go up and down through these stages all night, until we wake in the morning.
Hello, curious kids! Have you got a question you'd like an expert to answer? Ask an adult to send your question to us. They can: * Email your question to curiouskids@theconversation.edu.au
* Tell us onTwitter by tagging @ConversationEDU with the hashtag #curiouskids, or
* Tell us onFacebook

CC BY-ND

Please tell us your name, age, and which city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! We won't be able to answer every question but we will do our best.

# data[].attributes.attachment_link.title
Curious Kids: What happens in our bodies when we sleep?

# data[].attributes.attachment_link.description
This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they'd like an expert to answer. All questions are welcome - serious, weird or wacky! Hi. My name is Abhilasa, I am 9 years old and I am from Melbourne and my question is: What happens in our bodies when we sleep?

# data[].attributes.attachment_link.readable_text
So waves are everywhere. But what makes a wave a wave? What characteristics, properties, or behaviors are shared by the phenomena that we typically characterize as being a wave? How can waves be described in a manner that allows us to understand their basic nature and qualities?

A wave can be described as a disturbance that travels through a medium from one location to another location. Consider a slinky wave as an example of a wave. When the slinky is stretched from end to end and is held at rest, it assumes a natural position known as the equilibrium or rest position. The coils of the slinky naturally assume this position, spaced equally far apart. To introduce a wave into the slinky, the first particle is displaced or moved from its equilibrium or rest position. The particle might be moved upwards or downwards, forwards or backwards; but once moved, it is returned to its original equilibrium or rest position. The act of moving the first coil of the slinky in a given direction and then returning it to its equilibrium position creates a disturbance in the slinky. We can then observe this disturbance moving through the slinky from one end to the other. If the first coil of the slinky is given a single back-and-forth vibration, then we call the observed motion of the disturbance through the slinky a slinky pulse. A pulse is a single disturbance moving through a medium from one location to another location. However, if the first coil of the slinky is continuously and periodically vibrated in a back-and-forth manner, we would observe a repeating disturbance moving within the slinky that endures over some prolonged period of time. The repeating and periodic disturbance that moves through a medium from one location to another is referred to as a wave.

What is a Medium?

But what is meant by the word medium? A medium is a substance or material that carries the wave. You have perhaps heard of the phrase news media. The news media refers to the various institutions (newspaper offices, television stations, radio stations, etc.) within our society that carry the news from one location to another. The news moves through the media. The media doesn't make the news and the media isn't the same as the news. The news media is merely the thing that carries the news from its source to various locations. In a similar manner, a wave medium is the substance that carries a wave (or disturbance) from one location to another. The wave medium is not the wave and it doesn't make the wave; it merely carries or transports the wave from its source to other locations. In the case of our slinky wave, the medium through that the wave travels is the slinky coils. In the case of a water wave in the ocean, the medium through which the wave travels is the ocean water. In the case of a sound wave moving from the church choir to the pews, the medium through which the sound wave travels is the air in the room. And in the case of the stadium wave, the medium through which the stadium wave travels is the fans that are in the stadium.

Particle-to-Particle Interaction

To fully understand the nature of a wave, it is important to consider the medium as a collection of interacting particles. In other words, the medium is composed of parts that are capable of interacting with each other. The interactions of one particle of the medium with the next adjacent particle allow the disturbance to travel through the medium. In the case of the slinky wave, the particles or interacting parts of the medium are the individual coils of the slinky. In the case of a sound wave in air, the particles or interacting parts of the medium are the individual molecules of air. And in the case of a stadium wave, the particles or interacting parts of the medium are the fans in the stadium.

Consider the presence of a wave in a slinky. The first coil becomes disturbed and begins to push or pull on the second coil; this push or pull on the second coil will displace the second coil from its equilibrium position. As the second coil becomes displaced, it begins to push or pull on the third coil; the push or pull on the third coil displaces it from its equilibrium position. As the third coil becomes displaced, it begins to push or pull on the fourth coil. This process continues in consecutive fashion, with each individual particle acting to displace the adjacent particle. Subsequently, the disturbance travels through the medium. The medium can be pictured as a series of particles connected by springs. As one particle moves, the spring connecting it to the next particle begins to stretch and apply a force to its adjacent neighbor. As this neighbor begins to move, the spring attaching this neighbor to its neighbor begins to stretch and apply a force on its adjacent neighbor.

A Wave Transports Energy and Not Matter

When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles that restores them to their original position. In a slinky wave, each coil of the slinky ultimately returns to its original position. In a water wave, each molecule of the water ultimately returns to its original position. And in a stadium wave, each fan in the bleacher ultimately returns to its original position. It is for this reason, that a wave is said to involve the movement of a disturbance without the movement of matter. The particles of the medium (water molecules, slinky coils, stadium fans) simply vibrate about a fixed position as the pattern of the disturbance moves from one location to another location.

Waves are said to be an energy transport phenomenon. As a disturbance moves through a medium from one particle to its adjacent particle, energy is being transported from one end of the medium to the other. In a slinky wave, a person imparts energy to the first coil by doing work upon it. The first coil receives a large amount of energy that it subsequently transfers to the second coil. When the first coil returns to its original position, it possesses the same amount of energy as it had before it was displaced. The first coil transferred its energy to the second coil. The second coil then has a large amount of energy that it subsequently transfers to the third coil. When the second coil returns to its original position, it possesses the same amount of energy as it had before it was displaced. The third coil has received the energy of the second coil. This process of energy transfer continues as each coil interacts with its neighbor. In this manner, energy is transported from one end of the slinky to the other, from its source to another location.

This characteristic of a wave as an energy transport phenomenon distinguishes waves from other types of phenomenon. Consider a common phenomenon observed at a softball game - the collision of a bat with a ball. A batter is able to transport energy from her to the softball by means of a bat. The batter applies a force to the bat, thus imparting energy to the bat in the form of kinetic energy. The bat then carries this energy to the softball and transports the energy to the softball upon collision. In this example, a bat is used to transport energy from the player to the softball. However, unlike wave phenomena, this phenomenon involves the transport of matter. The bat must move from its starting location to the contact location in order to transport energy. In a wave phenomenon, energy can move from one location to another, yet the particles of matter in the medium return to their fixed position. A wave transports its energy without transporting matter.

Waves are seen to move through an ocean or lake; yet the water always returns to its rest position. Energy is transported through the medium, yet the water molecules are not transported. Proof of this is the fact that there is still water in the middle of the ocean. The water has not moved from the middle of the ocean to the shore. If we were to observe a gull or duck at rest on the water, it would merely bob up-and-down in a somewhat circular fashion as the disturbance moves through the water. The gull or duck always returns to its original position. The gull or duck is not transported to the shore because the water on which it rests is not transported to the shore. In a water wave, energy is transported without the transport of water.

The same thing can be said about a stadium wave. In a stadium wave, the fans do not get out of their seats and walk around the stadium. We all recognize that it would be silly (and embarrassing) for any fan to even contemplate such a thought. In a stadium wave, each fan rises up and returns to the original seat. The disturbance moves through the stadium, yet the fans are not transported. Waves involve the transport of energy without the transport of matter.

In conclusion, a wave can be described as a disturbance that travels through a medium, transporting energy from one location (its source) to another location without transporting matter. Each individual particle of the medium is temporarily displaced and then returns to its original equilibrium positioned.

Check Your Understanding

1. TRUE or FALSE:

In order for John to hear Jill, air molecules must move from the lips of Jill to the ears of John.

2. Curly and Moe are conducting a wave experiment using a slinky. Curly introduces a disturbance into the slinky by giving it a quick back and forth jerk. Moe places his cheek (facial) at the opposite end of the slinky. Using the terminology of this unit, describe what Moe experiences as the pulse reaches the other end of the slinky.

3. Mac and Tosh are experimenting with pulses on a rope. They vibrate an end up and down to create the pulse and observe it moving from end to end. How does the position of a point on the rope, before the pulse comes, compare to the position after the pulse has passed?

4. Minute after minute, hour after hour, day after day, ocean waves continue to splash onto the shore. Explain why the beach is not completely submerged and why the middle of the ocean has not yet been depleted of its water supply.

5. A medium is able to transport a wave from one location to another because the particles of the medium are ____.

a. frictionless
b. isolated from one another

c. able to interact

d. very light

# data[].attributes.attachment_link.keywords[]
how is a wave different than a vibration

# data[].attributes.attachment_link.description
What makes a wave a wave? What characteristics, properties, or behaviors are shared by the phenomena that we typically characterize as being a wave? How can waves be described in a manner that allows us to understand their basic nature and qualities?

# data[].attributes.body
Radio Waves

# data[].attributes.attachment_link.readable_text
In November 1886 Heinrich Hertz became the first person to transmit and receive controlled radio waves.

Considering how indispensable his wireless transmissions quickly became, it seems a little odd looking back that he had no practical purpose in mind for the radio or Hertzian waves he discovered.

His research was focused solely on discovering if James Clerk Maxwell's 1864 theory of electromagnetism was correct.

A Young Man in a Hurry

The first time Hertz thought seriously about proving Maxwell's theory was in 1879, when he was a 22 year-old student in Berlin. He decided against it. It seemed too hard, and anyway he wanted to concentrate on completing his doctorate.

In 1883, after getting his first lecturing job, he revisited Maxwell's theory. He wrote an impressive paper, reworking the theory mathematically.

In 1885 he moved to the University of Karlsruhe as a full professor of experimental physics. Now he decided the time was ripe to look for a way to prove Maxwell's theory.

(We have more details of Hertz's life here.)

A Spark of Genius

In October 1886 Hertz saw an electrical spark, starting a train of thought that would end up transforming the world.

Hertz had been demonstrating a piece of electrical apparatus called Riess spirals to students. The spirals produced electric sparks by a process called magnetic induction. The sparks flew between spark-gaps - small gaps in circuits.

Hertz became fascinated by sparks.

He started generating them using a piece of electrical equipment called an induction coil. (A car's spark plugs are powered by an induction coil. The induction coil transforms low voltage dc electricity coming from a car's battery into high voltage ac electricity. This electricity crosses a small air gap at regular intervals as a spark - i.e. you have a spark plug.)

You can see a diagram of an induction coil connected to a spark-gap below.

Playing around a little with this apparatus, Hertz connected a secondary spark-gap to the existing spark-gap, as shown.

He used the induction coil to generate high voltage ac electricity, producing a series of sparks at regular intervals at the main spark-gap.

Hertz found that when sparks flew across the main gap, sparks also usually flew across the secondary gap - that is between points A and B in the image; Hertz called these side-sparks.

He found the behavior of the side-sparks highly thought-provoking.

He varied the position of connection point C on the side-circuit. The only way he could stop side-sparks being produced was to arrange the apparatus so the length of wire CA was the same as CB.

Given that the electricity was ac, this suggested to Hertz that voltage waves were separately racing through the wire along paths CA and CB.

If the distances CA and CB were the same, then the same voltage must reach points A and B at the same time. The electrical waves in CA and CB were said to be in phase with one another, so sparks could not be generated. Sparks could only be generated if there was a large voltage difference between points A and B.

Perfectly Behaved Electric Waves

Hertz did more experiments which revealed that the sparking at the main gap was producing beautifully regular electrical waves, whose behavior was predictable.

He pictured waves of electric charge moving back and forth, creating a standing wave within the wire.

In other words, he believed the circuit was vibrating like a tuning fork at its natural, resonant frequency. He thought he now had a circuit in resonance.

Of course, in Hertz's circuit the vibrations were not of sound, they were vibrations of electric charge.

It's worth bearing in mind that resonance is not actually needed for electromagnetic waves to be produced - they're produced whenever electric charges are accelerated.

The importance of resonance is that if a receiver has the same resonant frequency as a transmitter, the incoming electromagnetic waves have a much stronger effect on it. This is similar to the situation in which an opera singer shatters a champagne glass because its resonant frequency is the same as the note she sings.

Aware that the frequency of electrical vibrations and hence resonance is determined by electrical properties called inductance and capacitance, Hertz looked more closely at these factors in the circuit.

Breaking Away

He identified that a phenomenon called self-induction was taking place in the wires. This allowed him to deduce that the electric vibrations had an extraordinarily high frequency.

Hertz decided to break the hard-wired connection between the main spark circuit and the side-spark circuit, as shown in the image.

He also arranged the capacitance and inductance of the main circuit so its resonant frequency was 100 million times a second. Today we would write this vibration frequency as 100 MHz. (The unit of frequency is, of course, the hertz (Hz), named in Heinrich Hertz's honor.)

According to Maxwell's theory, the main circuit would then radiate electromagnetic waves with a wavelength of about a meter.

The actual apparatus is shown below.

Producing and Detecting Radio Waves

In November 1886 Hertz put together his spark-gap transmitter, which he hoped would transmit electromagnetic waves.

For his receiver he used a length of copper wire in the shape of a rectangle whose dimensions were 120 cm by 80 cm. The wire had its own spark-gap.

Hertz applied high voltage a.c. electricity across the central spark-gap of the transmitter, creating sparks.

The sparks caused violent pulses of electric current within the copper wires leading out to the zinc spheres.

As Maxwell had predicted, the oscillating electric charges produced electromagnetic waves - radio waves - which spread out at the speed of light through the air around the wire.

Hertz detected the waves with his copper wire receiver - sparks jumped across its spark gap, even though it was as far as 1.5 meters away from the transmitter. These sparks were caused by the arrival of electromagnetic waves from the transmitter generating violent electrical vibrations in the receiver.

This was an experimental triumph. Hertz had produced and detected radio waves.

Strangely, though, he did not appreciate the monumental practical importance of his discovery.

In fact Hertz's waves would soon change the world. By 1896 Guglielmo Marconi had been granted a patent for wireless communications. By 1901 he had made a wireless transmission across the Atlantic Ocean from Britain to Canada.

By the early 1900s technically minded people were building their own spark transmitters at home. Even children got in on the act, with instructions to build a transmitter appearing in a craft book for boys in 1917.

A 'Build at Home' Spark-Gap Transmitter

Goodbye to Sparks
By the late 1920s most radio transmitters were using vacuum tubes rather than sparks to generate radio waves. And then the vacuum tubes were abandoned in favor of transistors.

Scientists and engineers have continued to innovate quickly in the field of radio technology. Radio, television, satellite communications, mobile phones, radar, and many other inventions and gadgets have made Hertz's discovery an indispensable part of modern life.

Author of this page: The Doc
Images digitally enhanced and colorized by this website. © All rights reserved.

Further Reading
Heinrich Hertz
Electric Waves
Macmillan and Co., 1893

# data[].attributes.attachment_link.title
How Heinrich Hertz Discovered Radio Waves

# data[].attributes.attachment_link.description
In November 1886 Heinrich Hertz became the first person to transmit and receive controlled radio waves. Considering how indispensable his wireless transmissions quickly became, it seems a little odd looking back that he had no practical purpose in mind for the radio or Hertzian waves he discovered.

# data[].attributes.body
Wave basics

# data[].attributes.attachment_link.title
Physics - Waves -  Introduction - Definitions, Logintudinal and Transverse.

# data[].attributes.body
Medicine: Ultrasound Technology

# data[].attributes.body
Seismic Waves

# data[].attributes.attachment_link.title
How Earthquake occurs and what causes it | Seismic Waves | P and S Waves

# data[].attributes.body
History: Breaking the Sound Barrier

# data[].attributes.attachment_link.title
Chuck Yeager Breaks the Sound Barrier -- X-1 -- 1947

# data[].attributes.attachment_link.readable_text
You might have enjoyed that ripple pattern formed in the pond after a stone is thrown in it. Also, you might have observed an upward and downward motion of a rope, when you shake it from one of its ends. These ripples and the upward and downward motion of the rope are the examples of a transverse wave.

Transverse waves are the waves in which the particles move in a direction perpendicular to that of a wave. The highest point is called "Crest," and the lowest point is called "Trough." They can even travel in vaccum.

Let's analyze some daily life examples of transverse waves.

Lightwave forms the most important example of a transverse wave. In case of the light waves, particles travel perpendicular to the direction of the waves. It is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.

2. Ripples in Water Pond

Whenever we throw a pebble into the pond, we see the circular ripples formed on its surface which disappear gradually. The water moves up and down, and the effect, ripple, which is visible to us looks like an outwardly moving wave.

3. Plucking a Guitar String

When you pluck the string of a guitar, the strings move up and down, exhibiting transverse wave. The sound wave is a longitudinal wave, but the wave on the guitar is, however, a transverse wave. The particles in the string move perpendicular to the direction of the wave propagation.

4. Earthquake (Seismic S wave)

The bump or rattle that we feel during an earthquake is due to seismic-S wave. It moves rock particles up and down, perpendicular to the direction of the wave propagation. These S-waves are transverse waves and require a solid medium to travel.

5. Tsunami Waves

Though Tsunamis have properties of both the transverse as well as a longitudinal wave; however, it starts as a transverse wave. It is transverse in form from the place where the earthquake starts under water and turns longitudinal when it reaches the shore.

6. X-Rays

Most of us might have undergone X-ray at one point or the other. These are the type of electromagnetic radiation in which the electric field and magnetic field propagate perpendicular to each other.

When a large number of spectators are attending a match or a game in a stadium, you might have noticed a metachronal rhythm achieved by successive groups of the audience when they stand, yell, or raise their arms. It is also called stadium wave or Mexican waves.

8. Electromagnetic Waves

We all experience electromagnetic waves daily even without knowing it; in radios, TV's, while cooking in microwaves and in hospitals (X rays and MRI). These are the transverse waves in which the electric field and magnetic field are perpendicular to each other.

9. Oscillating String or Rope

Most of us have experienced waving a rope up and down, while the other end is fixed. When we pull a rope or string from one end and keep the other end fixed, it exhibits a transverse wave.

# data[].attributes.attachment_link.description
You might have enjoyed that ripple pattern formed in the pond after a stone is thrown in it. Also, you might have observed an upward and downward motion of a rope, when you shake it from one of its ends. These ripples and the upward and downward motion of the rope are the examples of a transverse wave.

# data[].attributes.body
Wave Motion

# data[].attributes.attachment_link.title
The Doppler Effect: what does motion do to waves?

# data[].attributes.attachment_link.readable_text
Have you ever noticed a slinky or a spring waving back and forth, just like shown below? This type of motion in which particles move along the direction of the wave back and forth is a longitudinal motion.
Longitudinal waves are the waves in which the particles of the medium propagates along the direction of the motion. Simply, particles travel along the direction of the motion or a wave. They are composed of compression (when particles/wave move close to each other) and rarefaction (when particles/wave move away). They require a medium to travel.
Let's explore the examples of Longitudinal wave in our daily life.

1. Speaking on the mic

A sound wave is a significant example of a longitudinal wave. When a speaker speaks some words in front of the microphone, he/she hit the air thousands of time per second at different frequencies. The sound particles travel along with the air particles and enter the mic to produce sound.

2. Clapping

When we clap while singing a birthday song or on any other occasion, do you know, what makes our hand produce that familiar sound of a clap. When we applaud, we compress and displace the air particles between our hands for a part of a second, which produces the sound of a clap we are familiar with.

3. Vibrating Drumheads

All of us are familiar with the sound of a drum, and most of us have also tried hitting the drum in different beats. When we hit the drum with the mallet, drum head vibrates and produce soundwaves. The soundwaves are generated because the drum head moves outward and inward, making air particles to move (vibrate) in the same direction.

4. Tsunami Waves

Tsunamis cause damage to coastal regions and that's why people residing in coastal areas are afraid of them. Most people think that sea waves are a transverse wave as they go up and down. However, sea waves, including Tsunami, are the example of both transverse as well as a longitudinal wave. When the waves reach the shore or smaller areas, they become smaller and thinner, and water particles move parallel to the wave, hence making it a longitudinal wave.

5. Earthquake (Seismic-P wave)

It is said that animals can sense the earthquake waves much before humans. They have the ability to sense the seismic P waves, which travel only in the interior of the earth. Even humans can feel a little bump and rattle of these waves, but they are mostly unnoticeable to us. The P waves are the fastest, and they require a medium to travel (solid and liquid). These waves cause the interior of the earth (tectonic plates) to move back and forth in a longitudinal manner, which leads to the surface waves (seismic S wave), which we can feel.

6. Vibration in Window Panels after a Thunder

Whenever it is raining heavily, and thunders are there, you might have noticed the vibration in window panels of your home; it happens because of sound waves. Lightning causes an increase in the air pressure and temperature, which creates a shock wave of sound that we hear like a loud boom and cause our window panels to vibrate.

7. Music Woofers

Have you ever noticed the movement of the woofer cone; moving in and out or ever felt air pressure on your hand when you try to cover the mouth of a woofer? It's because woofers work on the phenomenon of a longitudinal wave. They move the air particles in or out, producing sound.

# data[].attributes.attachment_link.title
7 Real Life Examples Of Longitudinal Waves

# data[].attributes.attachment_link.description
Have you ever noticed a slinky or a spring waving back and forth, just like shown below? This type of motion in which particles move along the direction of the wave back and forth is a longitudinal motion. Longitudinal waves are the waves in which the particles of the medium propagates along the direction of the motion.

# data[].attributes.attachment_link.title
Physics Waves: Frequency & Wavelength FREE Science Lesson

# data[].attributes.body
History: Breaking the Sound Barrier

# data[].attributes.body
Wave motion

# data[].attributes.attachment_link.title
Wave Motion | Waves | Physics | FuseSchool