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Physics A-Level > Chapter 3: Waves > Flashcards

Chapter 3: Waves Flashcards

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1
Q

How does a wave transfer energy through a medium?

A

By causing the particles in the medium to oscillate.

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2
Q

Which of the following measurements can take a negative value?
1. Displacement
2. Amplitude

A

Displacement only.

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3
Q

State what units the following properties of waves are measured in:
1. Displacement
2. Amplitude
3. Frequency

A
  1. Metres
  2. Metres
  3. Hertz
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4
Q

What is the phase of a wave.

A

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

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5
Q

What is the phase difference of two waves?

A

The amount by which one wave lags behind another.

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6
Q

How would you calculate the frequency of a wave, given its period?

A

Frequency = 1/period

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7
Q

What does c stand for in the equation c = fλ?

A

Wave speed

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8
Q

What can you say about the speed of electromagnetic waves with different frequencies travelling in a vacuum?

A

They all travel at the speed of light — 3.0x10^8 ms^-1

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9
Q

What is the difference between transverse and longitudinal waves?

A

In transverse waves, the particles/fields oscillate perpendicular to the propagation of energy. In longitudinal waves, the particles/fields oscillate parallel to the propagation of energy.

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10
Q

Give an example of a transverse wave and a longitudinal wave.

A

Transverse: - electromagnetic, seismic-s waves
Longitudinal: - sound waves, seismic-p waves

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11
Q

What happens when you put two polarising filters at right angles in front of a beam of light?

A

No light passes through.

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12
Q

What happens to unpolarised when it is reflected from the surface of water?

A

It is partially polarised — some of the vibrations of the reflected light are in the same direction.

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13
Q

Explain how polaroid sunglasses reduce glare.

A

Light is partially polarised when reflected by some materials. Polaroid sunglasses block out light in which the reflected light is partially polarised, but let through light vibrating in other directions. This reduces glare without reducing visibility.

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14
Q

Other than polarising sunglasses, give one example of how polarised waves are relevant to everyday life.

A

— Reducing reflections in photography
— Aligning TV and radio receivers

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15
Q

What does the principle of superposition say?

A

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

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16
Q

Describe constructive interference.

A

When two waves pass through each other and their displacements combine to make a displacement with greater magnitude.

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17
Q

What is total destructive interference?

A

When two waves pass through each other and their displacements cancel each other out completely.

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18
Q

What is the phase difference of two points on a wave?

A

The phase difference of two points on a wave is the difference in their positions in the wave’s cycle.

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19
Q

Give three possible units for phase difference.

A
  1. Degrees
  2. Radians
  3. Fractions
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20
Q

When are two points on a wave exactly out of phase?

A

When their phase difference is an odd multiple of 180 degrees.

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21
Q

What does it mean for two waves to be in phase?

A

Two waves are in phase if they have a phase difference of 0 degrees or a multiple of 360 degrees.

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22
Q

How is a stationary wave formed?

A

When two progressive waves are travelling in opposite directions with the same frequency and amplitude, their superposition creates a stationary wave.

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23
Q

Does a stationary wave transfer energy?

A

No

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24
Q

Describe what a resonant frequency of a string is.

A

A resonant frequency is a frequency at which a stationary wave is formed because an exact number of waves are produced in the time it takes for a wave to get to the end of the string and back again.

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25
Q

Give an example of a way to observe:
1. Stationary sound waves
2. Stationary microwaves

A
  1. Use a loudspeaker to direct sound waves into a glass tube with a flat end to reflect them. Put powder in the tube and watch as it collects at the nodes.
  2. Use metal plate to reflect microwaves and a probe to observe nodes and antinodes.
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26
Q

Describe and briefly explain an experimental set-up used to investigate the factors which affect the resonant frequencies of a stretched string.

A

A string is fixed at one end to a vibration transducer. The other end runs over a pulley and has masses attached to it. This set up can be used to investigate how length, mass per unit length and tension on a string affect its resonant frequency. Masses can be added to the end of the string to change the tension on the string. Different types of string can be used to vary the mass per unit length and the vibration transducer can be moved with respect to the pulley to vary string length.

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27
Q

What would happen to the frequency of the first harmonic of a string if the string was replaced with a heavier string of the same length and everything else was kept constant? Explain why this is the case.

A

The resonant frequency of the string would decrease if a heavier string was used, as the mass per unit length would increase. The decrease in frequency is because waves travel slower down a heavier string.

28
Q

A vibration transducer is used to create stationary waves on a string. Explain why increasing the length of the vibrating string causes the frequency of the first harmonic to decrease.

A

The harmonic frequencies correspond to the number of half wavelengths on a string. If the length of a string is increased, the wavelength of the resonant frequency also increases so the frequency must decrease.

29
Q

State the equation for calculating the frequency of the first harmonic of a stretched string.

A

f = 1/2L x √T/μ

30
Q

What sort of waves interact?

A

All waves

31
Q

What size of gap would you expect to produce the most diffraction?

A

A gap whose size is roughly the same as the wavelength of the wave being diffracted.

32
Q

What size of gap would you expect to produce the most diffraction?

A

A gap whose size is roughly the same length as the wavelength of the wave being diffracted.

33
Q

What is monochromatic light?

A

Monochromatic light is light made up of only one frequency (and wavelength).

34
Q

What sort of interference is responsible for the bright fringes on a diffraction pattern produced by laser light passing through a single slit?

A

Constructive interference

35
Q

Describe the diffraction pattern produced when white light is shone through a single narrow slit.

A

A white central maximum with outer fringes that are spectra.

36
Q

What property of laser light means that it will produce a clearer diffraction pattern than white light? Explain your answer.

A

Laser light is monochromatic, which means all of the light is the same colour. White light is made up of a range of wavelengths which diffract by different amounts, so it doesn’t produce a very clear diffraction pattern. Monochromatic light is only made up of one wavelength, so the pattern is much clearer.

37
Q

Explain what would happen to the central maximum of a single-slit diffraction pattern if the slit width was decreased.

A

If the width of the slit was decreased, then the amount of diffraction would increase. This would cause the central maximum to get wider and less intense.

38
Q

Explain what effect increasing the wavelength of a light source would have on the width of the central maximum of its single-slit diffraction pattern.

A

If the wavelength of the incident light was increased, then the amount of diffraction would increase. This would cause the central maximum to get wider and less intense.

39
Q

Explain what happens in terms of photons when the intensity of a monochromatic light source is increased.

A

Intensity is the power per unit area. Monochromatic light is made up of photons which all have equal energies. This means that an increase in intensity results in more photons hitting a unit area in a given time.

40
Q

What does it mean for two wave sources to be coherent?

A

Two wave sources are coherent if the waves have the same wavelength and frequency and a fixed phase difference between them.

41
Q

What must be true of two wave sources if they produce a clear, standard two-source interference pattern?

A

They must be coherent.

42
Q

What is meant by the path difference between two waves?

A

The path difference is the amount by which the path travelled by one wave is longer than the path travelled by the other wave.

43
Q

At what path differences will you see constructive interference?

A

You can see constructive interference when the path difference equals nλ.

44
Q

How can you create two coherent sources of sound waves?

A

Have one amplifier attached to two loudspeakers.

45
Q

Describe an experiment to produce and observe an interference pattern with sound waves.

A

Attach two loudspeakers to an amplifier and create sound waves at a set frequency. Walk along a straight line parallel to the line of the speakers. Mark down points of maximum loudness and quietness, which show the position of constructive and destructive interference.

46
Q

Explain what you would observe if you moved a microwave probe in a straight line parallel to the line of two coherent microwave transmitters.

A

The probe would detect alternating areas of maximum and minimum signal strength.

47
Q

How can you create two coherent sources of light waves?

A

By shining light through a double-slit system.

48
Q

Why should you never look directly at a laser beam?

A

It is dangerous as your eye’s lens would focus the beam into your retina, which would then be permanently damaged.

49
Q

Write down Young’s double-slit formula which links fringe spacing (w), wavelength (λ), distance between slits (s) and distance between slits and screen (D).

A

w = λD/s

50
Q

Describe an experiment to find how the wavelength of a laser light source affects the fringe spacing of its interference pattern through a double slit.

A

Mount a card with two thin slits in a distance D from an observation screen. Shine a laser beam through the slits onto the screen, then measure the fringe spacing on the observation screen using a ruler. Use a variety of different laser sources to vary wavelength, whilst measuring the fringe spacing for each. Use your results to plot a graph of wavelength against fringe spacing.

51
Q

Explain how Young’s double-slit experiment suggested that light was a wave.

A

Young’s double slit experiment showed that light could diffract and interfere. Nothing of these qualities are wave properties, which suggested light was a wave.

52
Q

Why is it often better to use a diffraction grating instead of a double-slit set-up?

A

The fringes produced are much sharper.

53
Q

What’s the zero order line of a diffraction grating experiment?

A

Line of maximum brightness at the centre of a diffraction pattern. Same direction as the incident beam.

54
Q

What would happen to the interference pattern produced if you increased the wavelength of light transmitted through a diffraction grating?

A

The pattern would spread out.

55
Q

What would happen to the interference pattern produced if the light was transmitted through a coarser diffraction grating?

A

The pattern would be less spread out.

56
Q

What is the refractive index of a material?

A

A measure of the optical density of the material, given by the ratio of the speed of light in the material.

57
Q

What if the formula for finding the refractive index?

A

n=c/cs

58
Q

What is the relative refractive index? Formula for two materials at a boundary.

A

The relative refractive index between two material, 1n2, is the ratio of the speed of light in material one to the speed of light in material 2. 1n2 = n2/n1

59
Q

Light travels from one material to another and refracts at the boundary. If you know the angle of incidence, the angle of refraction and the refractive index of the first material, how would you find the refractive index of the second material?

A

Use the law of refraction. n1sinθ1 = n2sinθ2
And rearrange.

60
Q

In what way will light bend if it passes at an angle into a medium with a higher refractive index than the material if just left?

A

It will bend towards the normal.

61
Q

What do we mean by the critical angle of a boundary of two materials?

A

The critical angle of a boundary is the angle of incidence at which the angle of refraction is 90 degrees.

62
Q

What conditions need to be met for total internal reflection?

A

n2<n1 and the angle of incidence must be greater than the critical angle.

63
Q

State two functions of the cladding of optical fibres.

A

Cladding has a lower optical density that the optical fibre, so it allows total internal reflection. It is also used to protect the fibre from scratches and damage which could let light escape.

64
Q

Explain how absorption in optical fibres causes signal degradation.

A

Absorption is where the fibre material absorbs some of the energy from the signal, which reduces the amplitude of the signal, leading to signal degradation.

65
Q

Name two kinds of dispersion that can cause pulse broadening in optical fibres and explain what causes them.

A

Modal dispersion is caused by light rays taking different paths down the fibre, some of which are faster than others. This is caused by light entering the fibre at a range of angles. Material dispersion is caused by different wavelengths of the fibre. This leads to different wavelengths taking different times to travel through the fibre.

66
Q

Explain why signal degradation is a problem when using optical fibres to send information.

A

It can result in a loss of information.

67
Q

Describe and explain three ways to prevent signal degradation in optical fibres.

A

— Using a single-mode fibre reduces modal dispersion, as light is restricted to a very narrow path.
— Using monochromatic light prevents material dispersion as the light is only made up of one wavelength.
— Using a signal booster to regenerate and amplify the original signal regularly prevents losses from absorption and dispersion.

Physics A-Level (10 decks)

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