WAVES: LIGHT&SOUND by Lizelle Swanepoel

WHAT IS A WAVE?

lizellexs — 2019-02-14 03:57:05 UTC

Wave = A succession of pulses.
Pulse = A single disturbance traveling through a medium.
Medium = A substance or material through which a wave moves.

DEMO WITH A SLINKY.

Two types of waves

lizellexs — 2019-02-14 04:14:29 UTC

Transverse waves and Longitudinal waves:

Transverse wave - The disturbance that makes up the wave, where the particles disturbed by the wave move perpendicular to the direction of the wave. (e.g. Light, EM radiation, Water waves)
Longitudinal wave - The disturbance that makes up the wave where the particles disturbed by the wave move parallel to the direction of the wave (e.g. Sound). Known as a PRESSURE WAVE.

Energy flow through the wave, not matter.

lizellexs — 2019-02-14 04:18:56 UTC

Movement of particles in a transverse wave

lizellexs — 2019-02-14 04:21:50 UTC

Parts of a wave

lizellexs — 2019-02-14 04:27:30 UTC

THE WAVE EQUATION:

lizellexs — 2019-02-14 04:30:03 UTC

v = f.λ where, v is speed
f is frequency
λ is wavelength

HOW TO DERIVE THE WAVE EQUATION:
We know: speed = distance / time
For a wave: speed = wavelength / time
= λ / T
= λ x 1/T
= λ x f
v = fλ

Period vs Frequency of a wave

lizellexs — 2019-02-14 04:33:05 UTC

Superposition causing constructive or destructive interference.

lizellexs — 2019-02-15 04:41:47 UTC

lizellexs — 2019-02-15 04:49:37 UTC

lizellexs — 2019-02-15 04:50:49 UTC

REFRACTION EXPLAINED

lizellexs — 2019-02-22 08:10:40 UTC

Principle of Superposition

lizellexs — 2019-02-23 06:54:08 UTC

Period vs Frequency of a wave

lizellexs — 2019-02-23 06:58:05 UTC

Example 1:
The frequency of a wave is 20 Hz. Calculate its period.

Example 2:
The period of water waves is 12,5 seconds. What is its frequency?Period = the time taken for one complete wave to pass a fixed point.
Symbol for period = T
Unit = s (for seconds)

Frequency = the number of waves passing a given point in 1 second.
Symbol in equation = f
Unit = Hz (Hertz, s^-1 as in “per second”)

Frequency and period are inverses of each other.
Therefore, f = 1/T
and, T = 1/f

NOTES: p. 7
Example 1:
The frequency of a wave is 20 Hz. Calculate its period.

Example 2:
The period of water waves is 12,5 seconds. What is its frequency?

lizellexs — 2019-02-23 14:07:04 UTC

REFRACTION: Light changes direction in a different density medium

lizellexs — 2019-02-23 14:15:58 UTC

A light wave consists of individual photons which act as individual wave fronts, slowing down at the rate it enters the different medium.
This changes the direction of the light path.
From less dense to more dense media, the light path bends TOWARDS NORMAL LINE.
From more dense to less dense media, the light path bends AWAY FROM NORMAL LINE.

Longitudinal pulse

lizellexs — 2019-02-24 17:12:19 UTC

Transverse pulse

lizellexs — 2019-02-24 17:15:05 UTC

lizellexs — 2019-02-24 17:20:04 UTC

POLARISATION

lizellexs — 2019-02-24 17:29:01 UTC

Perpendicular motion of particles relative to direction of wave creates transverse waves.

lizellexs — 2019-02-24 19:26:18 UTC

OPTICS (LIGHT)

lizellexs — 2019-02-25 21:16:58 UTC

LIGHT = ELECTROMAGNETIC WAVES
Consists of 2 perpendicular waves:

a vertical electric field and
a horizontal magnetic field,
in phase with each other.

OPTICS (LIGHT WAVES)

lizellexs — 2019-02-25 21:26:48 UTC

Waves can be:

Reflected
Refracted
Diffracted
Superimposed (interference)

Descriptions:

Reflection of waves is the bouncing of waves off straight barriers; it follows the law of reflection.
Refraction is the change in direction of waves that occurs when waves travel from one medium to another. Refraction is always accompanied by a wavelength change and speed change.
Diffraction is the bending of waves around obstacles and openings.

Identify the wave phenomena below:

lizellexs — 2019-02-25 21:46:54 UTC

lizellexs — 2019-02-25 21:47:10 UTC

lizellexs — 2019-02-25 21:47:57 UTC

lizellexs — 2019-02-25 21:48:16 UTC

lizellexs — 2019-02-25 21:48:58 UTC

lizellexs — 2019-02-25 21:50:54 UTC

lizellexs — 2019-02-25 21:51:32 UTC

PINHOLE CAMERA

lizellexs — 2019-02-26 10:12:00 UTC

What characteristic of light explains why an inverted image of an object forms in a pinhole camera?

REFLECTION

lizellexs — 2019-02-26 10:15:13 UTC

NATURE OF LIGHT: rule 1

lizellexs — 2019-02-26 10:21:22 UTC

WAVES BOOKLET - EXTRA QUESTIONS

lizellexs — 2019-02-27 09:25:20 UTC

With Memo - old hard copy notes (work through what you can)

REFRACTION LAB PRAC - Law of Refraction (Snell’s Law)

lizellexs — 2019-02-28 22:08:54 UTC

Procedure:
1. Use a protractor to measure the angle of incidence (between ray and Normal line) and angle of refraction in each instance below:

lizellexs — 2019-02-28 22:10:03 UTC

2. Place the block on the sheet of paper and direct a ray streak onto the slit. Observe the ray tracks and measure the angles.

3. Change the angle with which the ray strikes the flat face, and record the angles of incidence and refraction again.

4. Repeat the process to collect a range of angles from zero,then 10 through 80 degrees.

lizellexs — 2019-02-28 22:12:45 UTC

5. Tabulate your results.

6. Calculate values of sin i/sin r for each pair of readings. Tabulate this in an extra column in your table.

7. Plot a graph of sin i against sin r. That is sin i on the y-axis and sin r on the x-axis.

8. Calculate the gradient of the graph. The value of the gradient of your graph is the refractive index.

Post prac questions

lizellexs — 2019-02-28 22:38:45 UTC

1. State the mathematical relationship emerging from this graph.

2. Do you think that water will refract light rays less or more than the Perspex block? Explain your answer.

3. Will the refractive index (n) of water be larger or smaller than n of the perspex block?

THE NATURE OF LIGHT

lizellexs — 2019-03-02 12:04:41 UTC

SNELL'S LAW

lizellexs — 2019-03-04 06:24:20 UTC

SNELL'S LAW

lizellexs — 2019-03-04 06:26:35 UTC

sin(i)/sin(r) = n
= n_{refractive medium}/n_airNOTE: n_air = 1n_airsin(i) = n_{refractive medium}sin(r)

n₁sin(i) = n₂sin(r) This is SNELL’S LAW

Also: n = c/v
= speed of light in a vacuum/ speed of light in different medium

CALCULATING WAVELENGTH

lizellexs — 2019-03-05 05:28:50 UTC

lizellexs — 2019-03-05 05:29:12 UTC

Predicting the position of a particle in a transverse wave in the next time interval, as the wave moves through the medium:

lizellexs — 2019-03-10 21:09:19 UTC

Does the particle move UP or DOWN next?

T.O.T.

lizellexs — 2019-03-12 10:57:07 UTC

TOTAL INTERNAL REFLECTION = the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.
CRITICAL ANGLE = the size of an incident angle that produces a refracted angle of 90°.

Optical fibers

lizellexs — 2019-03-12 10:59:24 UTC

TIR - APPLICATION IN MIRAGES

lizellexs — 2019-03-15 06:51:24 UTC

DRAWING RAY DIAGRAMS

lizellexs — 2019-03-18 08:25:45 UTC

REFLECTION

Some more definitions

lizellexs — 2019-03-25 06:39:03 UTC

Refractive index = the ratio of the speed of light in a vacuum to its speed in a refractive material, where bending of a ray of light occurs when passing from a medium into another with different density.
Optical density = a measure of the refracting power of a medium.
T.O.T. = the complete reflection of a light ray reaching an interface with a less dense medium, when the angle of incidence exceeds the critical angle.
Critical angle = The angle of incidence where the angle of refraction is 90°.
Normal = the line that is perpendicular to the plane of the surface.
c = 299 792 458 m.s^-1 ≃ 3 x 10⁸m.s^-1

ANSWERS - OPTICS EXERCISE 6

lizellexs — 2019-03-25 07:26:28 UTC

OLD NOTES

SOUND WAVES

lizellexs — 2019-03-27 21:33:36 UTC

Sound waves are pressure waves.

Pressure is force per unit area.
Hence, sound causes matter to vibrate.
Illustrated in video below:

How are sound waves produced?

lizellexs — 2019-03-27 21:42:47 UTC

Sound waves need a medium to travel through

lizellexs — 2019-03-27 21:43:22 UTC

Sound waves presented on an oscilloscope

lizellexs — 2019-03-27 21:43:45 UTC

An oscilloscope is a device that simply converts a sound wave (which is a longitudinal wave) into a transverse wave, for the purpose of making sound "visible".

Loudness vs. Pitch of Sound

lizellexs — 2019-03-27 21:44:08 UTC

Ampulitude of sound wave = Loudness
Frequency of sound wave = Pitch

lizellexs — 2019-03-27 21:44:29 UTC

lizellexs — 2019-03-27 21:44:49 UTC

Sound wave conversion on an Oscilloscope

lizellexs — 2019-03-27 22:10:21 UTC

Compressions translate to crests.
Rarefactions translate to troughs.
The louder a sound, the greater the amplitude of the wave.
The higher the pitch (“lowness our highness”) of the sound, the greater the frequency of the wave, i.e. waves get more squashed together.

NOTE:

SOUND WAVES = LONGITUDINAL WAVES (NOT TRANSVERSE WAVES!!)
An oscilloscope is an instrument that converts sound waves into electrical signals to show how the amplitude of sound waves changes with time.
An oscilloscope displays any longitudinal wave as a transverse wave.
•oscilloscope display: voltage – time
•microphone diaphragm vibrates, producing an a.c. waveform
Sound waves ARE NOT TRANSVERSE WAVES!

What is the audible range of sound?

lizellexs — 2019-03-27 22:13:29 UTC

For humans: from 20 Hz to 20000 Hz

Elephant: 16 Hz to 12,000 Hz

Cow: 16 Hz to 40,00 Hz

Cat: 100 Hz to 32000 Hz

Dog: 40 Hz to 46000 Hz

Rabbit: 1000 Hz to 1,00,000 Hz

ULTRASOUND = Sound with frequency > 20 000 Hz
INFRASOUND = Sound with frequency < 20 Hz
Ultra- and infrasound are inaudible, as they fall out of our audible hearing range.
Terms used for how fast an object MOVES:
- HYPERSONIC = speed of object ≥ Mach 5
- SUPERSONIC = Mach 1 < speed of object < Mach 5
- SUBSONIC = speed of object < Mach 1 (v_sound at spec. temperature)
- NOTE: Mach 1 = 343 m.s^-1

RESONANCE

lizellexs — 2019-03-29 05:37:55 UTC

Resonance is:
The increase in amplitude of oscillation of an electric or mechanical system exposed to a periodic force whose frequency is equal or very close to the natural frequency of the system.

Resonance is responsible for:

A glass breaking when you sing at it at its natural frequency
A bridge collapsing when wind blows over it and through the cables at the same frequency as the bridge's or cables’ natural frequency

What is a Sonic boom?

lizellexs — 2019-03-30 19:42:22 UTC

A sonic boom = a shock wave created by an object that travels faster than the speed of sound through air.

A sonic boom generates enormous amounts of sound energy
A sonic boom sounds like an explosion
If supersonic moving aircraft fly too close to buildings, it can break windows
If a person is exposed to a loud enough sonic boom, it could cause death (sonic boom can cause an embolism in the lungs which then travels to the heart)
The sound of the boom is heard before the sound of the plane

Sonic booms from the Falcon Heavy launch rocket boosters re-entry:

Breaking the sound barrier

lizellexs — 2019-03-30 19:49:21 UTC

When moving at v > v_sound,the sound barrier is broken.

Jet speed > 1200km/h, you will hear a sonic boom
Mach1 = 343 m/s = v _sound
Mach 2 = 686 m/s = 2v _sound
Mach number = the ratio of air speed to local speed of sound.
At altitude, Mach number is a function of temperature.

Speed of sound

lizellexs — 2019-03-30 20:14:27 UTC

Sound travels at:

331 m/s in air at 0 degrees C
343 m/s in air at room temperature
1 480 m/s in water (4.3 times as fast as in air)
5 120 m/s in iron (about 15 times as fast as in air)
12 000 m/s in exceptionally stiff material such as diamond (about 35 times as fast as in air) - maximum speed that sound will travel under normal conditions

ULTRASOUND IN SONARS

lizellexs — 2019-03-31 14:18:30 UTC

Echo Problem

lizellexs — 2019-03-31 14:32:17 UTC

Echolocation problem

lizellexs — 2019-03-31 14:36:02 UTC

ADDITIONAL QUESTIONS AND ANSWERS

lizellexs — 2019-04-05 12:21:39 UTC

lizellexs — 2019-04-06 18:00:14 UTC

lizellexs — 2019-04-06 18:00:15 UTC

lizellexs — 2019-04-06 18:07:16 UTC

Snell’s Law Calculations

lizellexs — 2019-04-06 18:13:54 UTC

n₁sinθ₁ = n₂sinθ₂where 1 = the incident medium
2 = the refracting medium

lizellexs — 2019-04-06 18:14:18 UTC

NOISE vs NON-NOISE

lizellexs — 2019-04-06 18:42:31 UTC

Noise = irregular and non-repeating pattern
Non-noise (musical instruments, singing, periodic speech voice) = regular and repeating pattern

SOUND = LONGITUDINAL WAVE

lizellexs — 2019-04-06 19:09:15 UTC

Sound is a pressure wave:

it needs a MEDIUM to propogate through
it cannot travel through a vacuum
it contains compressions (compressed areas)
it contains rarefactions (stretched out areas)
it's speed is 343 m.s^-1(in dry air at room temperature 20 degrees C)

NOTE:

We DRAW longitudinal waves using VERTICAL LINES:

Sonic boom explained

lizellexs — 2019-06-15 15:16:36 UTC

Wave speed EXERCISE

lizellexs — 2020-01-17 10:58:54 UTC

v = f λ

Page | 8
Example 3:
Waves with a frequency of 0,9 Hz are produced by a long rope. The wavelength of
the wave is 1,5 m. Calculate the speed of the wave.

Example 4:
The period of the waves produced by the spring is 0,5 seconds. The speed of the
wave is 6 m.s-1

4.1 What is the frequency of the waves?

4.2 Calculate the wavelength of the waves.

Page | 9
Example 5:
The slinky spring produces waves with a frequency of 3Hz and wavelength of 60cm.

5.1 Calculate the speed of the waves.

5.2 What will the wavelength be if the frequency is increased to 9 Hz?

DEFINITIONS

lizellexs — 2020-01-17 11:03:16 UTC

Wave: A succession of pulses.

Rest position: The equilibrium position of the medium before a pulse/wave is generated.

Oscillation: one complete swing or vibration.

Transverse pulse/wave : a pulse/wave that moves in a direction perpendicular to the direction of the disturbance in a medium.

Wavelength: the distance between two consecutive points on the wave that are in phase.

Amplitude: the maximum displacement of a particle from its rest (equilibrium)
position.

Period: the time taken for a wave to pass a point. The symbol for period is T, and its unit is second (s).

Frequency: the number of waves passing a point in one second. The symbol for frequency is f, and its SI unit is hertz (Hz).

Wave speed: the product of frequency and wavelength of a wave.

Longitudinal pulse/wave: a pulse/wave that moves in the same direction as the
disturbance in the medium.

Compression: a point on a medium through which a longitudinal wave is traveling that has the particles of the medium closely packed.

Rarefaction: a point on a medium through which a longitudinal wave is traveling that has the particles of the medium spaced apart.

Principle of superposition: States that when two disturbances occupy the same space at the same time, the resulting disturbance is the sum of two disturbances.

Photon: the smallest particle of light, which has no mass and which carries energy. (it behaves like both a wave and a particle)

The Law of Reflection:
1. The incident ray, the reflected ray and the normal all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.

Refraction: the change in direction of a wave when passing from one medium to
another due to the change in speed.

Optical density: the measure of the ease with which a material transmits light.

FORMULAE (SAGS)

lizellexs — 2020-01-17 11:06:54 UTC

f = 1/T
and
T = 1/f where, T is the period of a wave measured in second (s)
f is the frequency of a wave in hertz (Hz)

v = f λ where, v is the speed of the wave in m.s^-1 and λ is the wavelength in m.

c = f λ where, c is the speed of light = 3 x 10⁸ m.s⁻¹
f is the frequency measured in hertz (Hz)
λ is wavelength measured in metres (m)

n = c/v where, n is the refractive index (no units)
c is the speed of light in a vacuum in m.s^-1
v is the speed of light in medium in m.s^-1

Homework activity - Page 17

lizellexs — 2020-01-17 11:23:47 UTC

1. Define a longitudinal wave.

2. Draw a labelled diagram of a longitudinal wave.

3. A longitudinal wave travelling at 340 m.s-1 in air has a wavelength of 2,1m

3.1 What is meant by the term ‘wavelength’ with regard to a longitudinal
wave?

3.2 Calculate the frequency of the wave.

3.3 Calculate how far a compression will travel in one minute.

4. A certain wave has a frequency of 1900 Hz and wavelength of 18 cm.
Calculate the speed of the wave.

Amplitude

lizellexs — 2020-01-19 20:35:06 UTC

Theory page 6 - Make correction to amplitude on wave

lizellexs — 2020-01-23 08:19:09 UTC

The Oscilloscope

lizellexs — 2020-01-23 17:08:41 UTC

INDEX OF REFRACTION

lizellexs — 2020-01-23 17:39:37 UTC

Index of refraction of a medium is a measure of how dense it is.
Index of refraction = n = c / v_{light in the medium}

EM SPECTRUM

lizellexs — 2020-01-24 11:39:04 UTC

Radiation of all frequencies travels at the same velocity through space. The speed of light in vacuum, c, is equal to 3 x 10⁸ m.s⁻¹.

The wavelength and frequency of these waves can be calculated using the wave equation,
c = f λ
where c is the speed of light = 3 x 10⁸ m.s⁻¹
f is the frequency measured in hertz (Hz)
λ is wavelength measured in metres (m)

Dual nature of light

lizellexs — 2020-01-28 09:35:27 UTC

https://youtu.be/O81Cilon10M

LIGHT REFRACTION

lizellexs — 2020-01-28 09:42:12 UTC

When light passes from a less dense to a more dense substance, (for example passing from air into water), the light is refracted (or bent) towards the normal. The normal is a line perpendicular (forming a 90 degree angle) to the boundary between the two substances. ... As the light enters the water, it is refracted.

PHOTOELECTRIC EFFECT

lizellexs — 2020-01-28 09:51:19 UTC

Refraction ray diagram

lizellexs — 2020-01-31 10:50:24 UTC

Real vs apparent depth

EXERCISES

lizellexs — 2020-01-31 10:51:11 UTC

lizellexs — 2020-01-31 10:51:30 UTC

lizellexs — 2020-01-31 10:51:44 UTC

lizellexs — 2020-01-31 10:52:46 UTC

lizellexs — 2020-01-31 10:53:35 UTC

lizellexs — 2020-01-31 10:53:49 UTC

lizellexs — 2020-01-31 10:54:03 UTC

TOTAL INTERNAL REFLECTION AND CRITICAL ANGLE

lizellexs — 2020-01-31 11:03:32 UTC

lizellexs — 2020-10-27 21:36:31 UTC

Revision Worksheet

lizellexs — 2020-10-27 21:40:05 UTC

lizellexs — 2020-10-27 22:00:32 UTC

lizellexs — 2020-10-27 22:00:36 UTC