Section 3: Waves

Question 1

What is a progressive wave?

Answer 1

It carries energy from one place to another without transferring any material.

Question 2

How is a wave caused?

Answer 2

A wave is caused by something particles or fields oscillate at a source. These oscillations pass through the medium as the wave travels, carrying energy with it.

Question 3

How can we tell that waves carry energy?

Answer 3

Electromagnetic waves cause things to heat up.
X-rays and gamma rays knock electrons out of their orbits, causing ionisation.
Loud sounds cause large oscillations of air particles, which can make things vibrate.
Wave power can be used to generate electricity.

Question 4

What is reflection of a wave?

Answer 4

The wave is bounced back when it hits a boundary.

Question 5

What is refraction of a wave?

Answer 5

The wave changes direction as it enters a different medium. The change in direction is a result of the wave slowing down or speeding up.

Question 6

What is diffraction of a wave?

Answer 6

The wave spreads out as it passes through a gap or round an obstacle.

Question 7

What is meant by the displacement of a wave?

Answer 7

How far a point on the wave has moved from its undisturbed position.

Question 8

What is meant by the amplitude of a wave?

Answer 8

The maximum magnitude of displacement.
The distance from the undisturbed position to a crest or trough of the wave

Question 9

What is meant by the wavelength of a wave?

Answer 9

The length of one whole wave oscillation or wave cycle.
The distance between 2 adjacent points in phase on a wave.

Question 10

What is meant by the period of a wave?

Answer 10

The time taken from one whole wave cycle to go from its highest point and back to its highest point again.
The time taken for one complete oscillation or cycle.

Question 11

What is meant by the frequency of a wave?

Answer 11

The number of whole wave cycles (oscillations) per second passing a given point.

Question 12

What is meant by the phase of a wave?

Answer 12

A measurement of the position of a certain point along the wave cycle.

Question 13

What is meant by phase difference?

Answer 13

The amount by which one wave lags behind another wave.

Question 14

What can phase and phase difference be measured in?

Answer 14

In angles in degrees or radians.
In fractions of a cycle

Question 15

How are frequency and time period linked to each other?

Answer 15

f=1/T

Question 16

What is the unit for frequency?

Answer 16

Hertz ( Hz )

Question 17

What equations can you use to find wave speed?

Answer 17

v=d/t

c=f x lamda

Question 18

What is the speed of light in a vacuum?

Answer 18

3x10^8

Question 19

What is a method of measuring the speed of sound?

Answer 19

Use two microphones in a straight line a distance apart. The microphones should have separate inputs so the signals from each can be recorded separately.
Use the signal generator to produce a sound from the loudspeaker and use the computer to record the time between the first and second second microphone picking up the sound. Do this by measuring the time delay between the first peak of the signal received by each microphone in a graph of voltage against time.
You can then use speed=distance/time to calculate the soles of the sound waves.
You should repeat this experiment multiple times and take averages of your results.

Question 20

What is the method of measuring wave speed in water?

Answer 20

Set up a ripple tank.
Start by recording the depth of water in the tank using a ruler.
Use the ripple tank dipped to create vibrations with a regular frequency in the tank. Dim the main lights in the room and turn on the strobe light.
Increase the french if the strobe light from zero until the waves appear to be standing still. When this happens, the frequency of the strobe light is equal to the frequency of the water waves.
Use a ruler on the white paper below the tank to measure the distance between two adjacent peaks. You could make this measurement more precise by measuring the distance between several peaks and dividing this by the number of troughs in between. This distance between two adjacent peaks is equal to the wavelength so you can use the french and wavelength to find the speed.
Repeat this experiment for a range of water depths, measuring the wavelength and calculating the wave sped each time.
You should observe that the waves travel quicker in deeper water.

Question 21

What is a transverse wave?

Answer 21

The displacement of the particles is at right angles to the direction of energy propagation.

Question 22

What are some examples of transverse waves?

Answer 22

All electromagnetic waves
Ripples on water
Waves on strings
S-waves from earthquakes

Question 23

What are longitudinal waves?

Answer 23

The displacement of the particles is along the direction of energy propagation.

Question 24

What are some examples of longitudinal waves?

Answer 24

Sound
Primary waves from earthquakes

Question 25

What does a sound wave consist of in terms of shape?

Answer 25

Alternate compressions and rarefactions of the medium it’s travelling through.

Question 26

What is a polarised wave?

Answer 26

A wave that oscillates in one direction only.

Question 27

What is a polarising filter?

Answer 27

It can be used to polarise light and other waves. It only transits vibrations in one direction.
If you have two polarising filters at right angles to each other, then no light will get through.

Question 28

What type of wave does polarisation work on?

Answer 28

Polarisation can only happen for transverse waves.

Question 29

What provides evidence of the nature of transverse waves?

Answer 29

Polarisation.
In 1808, Etienne-Louise Malus discovered that light was polarised by reflection. At the time, light was thought of as a longitudinal wave so polarisation was hard to explain. In 1817, Young suggested that light was a transverse wave consisting of vibrating electric and magnetic fields perpendicular to the direction of energy transfer. This explained why light could be polarised.

Question 30

What are 4 applications of polarisation in the real world?

Answer 30

1. Glare reduction
2. Improving TV and radio signals
3. Stress analysis
4. Production and projection of 3D movies

Question 31

How is polarisation of light used to reduce glare?

Answer 31

When light reflected by surface such as water or glass enters the eye, it can cause glare. The fact that reflected light is partially polarised allows use to filter some of it out with polarising filters.
If you view partially-polarised reflected light through a polarising filter at the right angle, you can block out some of the reflected light, while still letting through light which vibrates at the angle of the filter. This reduces the intensity of the light entering your eye.

Question 32

How is polarisation used to improve TV and radio signals?

Answer 32

TV signals are polarised by the orientation of the rods on the transmitting aerial. To receive a strong signal, you have to line up the rods in the receiving aerial with the rods of the transmitting aerial - if they aren’t aligned, the signal strength will be lower, so the rods on TV aerials are all horizontal.

Question 33

When does superposition happen ?

Answer 33

When two or more waves pass through each other.
At the instant that waves cross, the displacements due to each wave combine. Then each wave continues on its way.

Question 34

What is the principle of superposition?

Answer 34

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.

Question 35

What are the types of interference you can get when two or more waves superpose?

Answer 35

Constructive
Destructive

Question 36

How is constructive interference caused?

Answer 36

When two waves meet, if their displacement are in the same direction, the displacements combine to give a bigger displacement.
A crest plus a crest gives a bigger crest.
A trough plus a trough gives a bigger trough.

Question 37

How is destructive interference caused?

Answer 37

If a wave with a positive displacement (crest) meet a wave with a negative displacement (trough), they will undergo destructive interference and cancel each other out.
The displacement of the combined wave is found by adding the displacements of the two waves.

Question 38

What is total destructive interference ?

Answer 38

If two waves with equal and opposite displacements meet, they cancel each other out completely.

Question 39

When are two points on a wave in phase?

Answer 39

If they both are at the same point in the wave cycles and have the same displacement and velocity.

Question 40

When two points have a phase difference of 0 or 360 degrees, what are they?

Answer 40

Completely in phase

Question 41

When two points on a wave have a phase difference of 180 degrees, what are they?

Answer 41

Completely out of phase.