Chapter 4 - Waves

Question 1

What are waves?

Answer 1

Oscillations of particles in a medium that transfer energy.

Question 2

What are progressive waves?

Answer 2

Waves that move - carrying energy from one place to another

Question 3

What 3 things can happen to waves?

Answer 3

Reflection, refraction and diffraction

Question 4

What is reflection?

Answer 4

When wave is bounced back when it hits a boundary e.g. you can see the reflection of light in mirrors.

Question 5

`What is refraction?

Answer 5

When wave changes direction when it enters a different medium. Change of direction is due to change of speed.

Question 6

What is diffraction?

Answer 6

When wave spreads out as it passes through a gap or round an obstacle. e.g. you can hear sound from round a corner

Question 7

What is the amplitude of a wave?

Answer 7

Maximum displacement from the undisturbed position. For transverse wave this is the height of a crest above equilibrium position. Measured in metres.

Question 8

What is the frequency of a wave?

Answer 8

Number of whole wave cycles (oscillations) per second passing a given point. Measured in Hertz

Question 9

What is the wavelength of a wave?

Answer 9

Length of one whole wave oscillation or wave cycle. Measured in metres.

Question 10

What is the phase of a wave?

Answer 10

Measurement of the position of a certain point along a wave cycle.

Question 11

What is the phase difference?

Answer 11

Amount by which one wave lags behind another wave.

Question 12

What are phase and phase difference measured in?

Answer 12

Measured in angles (degrees and radians) or fractions of a cycle.

Question 13

What is the time period of a wave?

Answer 13

Time taken for one whole wave cycle. Measured in seconds

Question 14

what is the equation linking frequency and time period

Answer 14

frequency = 1/ period

Question 15

What is the equation for wave speed?

Answer 15

c = frequency x wavelength

Question 16

Describe experiment to measure speed of sound

Answer 16

• Put two microphones in a straight line a distance ,d, apart. The microphones should have different 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 microphone picking up the sound. (Read off graph of voltage against time)
• Then use speed = distance/ time

Question 17

What is the speed of electromagnetic waves in a vacuum?

Answer 17

3.0 x 10^8 m/s

Question 18

What are transverse waves?

Answer 18

Waves where the direction of vibration is perpendicular to the direction of energy propagation.

Question 19

Give examples of transverse waves?

Answer 19

Electromagnetic waves, ripples of water, S seismic waves

Question 20

What are longitudinal waves?

Answer 20

• Waves where the direction of vibration of the particles is parallel to the direction of energy propagation.
• Consists of alternate compressions and rarefractions

Question 21

Give examples of longitudinal waves?

Answer 21

Sound waves and p seismic waves

Question 22

What is superposition?

Answer 22

When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.

Question 23

What is a polarised wave?

Answer 23

A wave that oscillates in one direction (plane)

Question 24

Which types of waves can be polarised?

Answer 24

Transverse waves - provides evidence for the nature of transverse waves.

Question 25

How does a polarising filter work?

Answer 25

Polarises unpolarised light, so that only light with vibrations in line with the filter’s transmission axis can pass through the first filter.

Question 26

What real world applications does polarisation have?

Answer 26

• Improving TV and radio signals
• sunglasses
• glare reduction

Question 27

How can polarisation improve Tv and radio signals?

Answer 27

By lining up the rods on the receiving rods with the rods on the transmitting aerial - if they aren’t aligned, the signal strength will be lower.

Question 28

How is polarisation used in polaroid sunglasses?

Answer 28

Partially polarised light reflected is polarised to reduce the intensity of the light entering person’s eye.

Question 29

What is constructive interference?

Answer 29

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

Question 30

What is destructive interference?

Answer 30

If a wave with positive displacement (crest) meets a wave with negative displacement (trough), they will undergo destructive interference and cancel each other out.

Question 31

What is total destructive interference?

Answer 31

If two waves with equal and opposite displacements meet, they cancel each other out. This is called total destructive interference.

Question 32

What is a stationary wave?

Answer 32

Superposition of two progressive waves with the same frequency (or wavelength) and amplitude, moving in opposite directions

Question 33

Describe the formation of a stationary wave by setting up a driving oscillator at one ned of a stretched string with the other end fixed?

Answer 33

Waves generated by oscillator reflect back and forth until a resonant frequency is reached where a wave and a reflected wave reinforce each other - forming a stationary waves that doesn’t move along just vibrates up and down, with nodes and antinodes.

Question 34

What is a node?

Answer 34

Points on a stationary wave where the amplitude of the vibration is zero.

Question 35

What is an antinode?

Answer 35

Points of maximum amplitude on a stationary wave.

Question 36

What is the equation linking amount of wavelengths that can fit on a string and the harmonic?

Answer 36

at the A th harmonic a/2 wavelengths will fit on the string.

Question 37

What is the first harmonic?

Answer 37

When the stationary wave is vibrating at its lowest possible resonant frequency.

Question 38

How can you produce a stationary wave using microwaves?

Answer 38

Reflecting a microwave beam at a metal plate - the superposition produces a stationary wave.
You can find the nodes and antinodes by moving the probe between the transmitter and reflecting plate. The meter or loudspeaker receives no signal at the nodes and maximum signal at the antinodes.

Question 39

Give 3 things that effect resonant frequency

Answer 39

• longer the string lower the resonant frequency
• The heavier (mass per unit length) the lower the resonant frequency
• lower the tension, lower the resonant frequency

Question 40

State some differences between transverse and longitudinal waves

Answer 40

• Oscillation of particles different direction

Question 41

In a stationary wave, what is the phase difference of the oscillations of particles at 2 adjacent antinodes?

Answer 41

180 degrees out of phase.