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AQA A-Level Physics > Waves (AS) > Flashcards

Waves (AS) Flashcards

1
Q

What is phase difference?

A

The amount one wave lags behind another.

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2
Q

What happens when a wave is reflected?

A

The wave is bounced back when it hits a boundary.

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3
Q

What happens when a wave is refracted?

A

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.

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4
Q

Are electromagnetic waves transverse or longitudinal?

A

Transverse.

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5
Q

In which direction do tranverse waves vibrate?

A

Perpendicular to the direction of energy transfer.

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6
Q

In which direction do longitudinal waves vibrate?

A

Same direction as energy transfer.

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7
Q

Are sound waves transverse or longitudinal?

A

Longitudinal.

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8
Q

What is a polarised wave?

A

A wave that only oscillates in one direction.

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9
Q

Can you polarise transverse waves?

A

Yes.

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10
Q

Can you polarise longitudinal waves?

A

No.

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11
Q

Polarisation is evidence that electromagnetic waves are ______. Physicists in 1808 thought that like was a _______ wave, and so were confused when light could be polarised. This suggested light was a ______ wave.

A

Polarisation is evidence that electromagnetic waves are (transverse). Physicists in 1808 thought that like was a (longitudinal) wave, and so were confused when light could be polarised. This suggested light was a (transverse) wave.

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12
Q

If you hold two polarising filters at ______ ______ to each other, no _____ will get through.

A

If you hold two polarising filters at (right angles) to each other, no (light) will get through.

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13
Q

Television and radio signals are ______. This is why you can ‘tune’ them, to align the waves accordingly.

A

Television and radio signals are (polarised). This is why you can ‘tune’ them, to align the waves accordingly.

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14
Q

What is superposition?

A

When two or more waves pass through each other, and so the displacement of each wave is combined to make one large wave.

‘Superposition’ means ‘one thing on top of another thing’.

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15
Q

A crest plus a crest gives a bigger crest. A trough plus a trough gives a bigger trough. What are these examples of?

A

Constructive interference.

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16
Q

A crest plus a trough (of equal sizes) cancel each other out. What is this an example of?

A

Deconstructive interference.

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17
Q

What is frequency?

A

Number of wave cycles occuring each second.

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18
Q

Two sources are coherent if they have the same ______ and ______ and a ______ ______ ______ between them.

A

Two sources are coherent if they have the same (wavelength) and (frequency) and a (fixed phased difference) between them.

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19
Q

What is a node?

A

A point of no displacement on stationary waves.

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20
Q

Are two points with a phase difference of zero or a multiple of 360° in phase?

A

Yes

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21
Q

Are two points with a phase difference of 180° and 360° in phase?

A

No, they’re exactly out of phase.

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22
Q

When do waves deconstructively interfere?

A

Waves deconstructively interfere when they are in antiphase with each other, i.e 180° out of phase.

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23
Q

When do waves constructively interfere?

A

Waves constructively interfere when they are in phase with each other.

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24
Q

What is a stationary (standing) wave?

A

A stationary (standing) wave is the superposition of two progressive waves with the same frequency (wavelength), moving in opposite directions.

A wave that stores, but does not transfer, energy.

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25
Q

What is an antinode?

A

A point of maximum displacement on a stationary wave.

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26
Q

What is the first harmonic?

A

A stationary wave vibrating at the lowest possible frequency.

Two nodes.

Has a length of half a wavelength.

27
Q

What is the second harmonic?

A

Twice the frequency of the first harmonic.

Three nodes.

Has a length of 1 wavelength.

28
Q

What is the third harmonic?

A

Three times the frequency of the first harmonic.

Four nodes.

Has a length of 1 1/2 wavelengths.

29
Q

The frequency of the first harmonic, f, is:
f = (1/2L)√(T/μ)
What does L, T and μ represent?

A

f = (1/2L)√(T/μ)

L = string length in m
T = tension in string in N
μ = mass per unit length of string in kgm^-1
μ = m/L 
T = mg
30
Q

In order to observe a clear diffraction pattern, what kind of light source do you need?

A

Monochromatic (same wavelength and frequency, so the same colour) and coherent light source.

Lasers are a monochromatic and coherent light source.

31
Q

What is diffraction?

A

Diffraction is the spreading of waves as they pass through a gap of a similar size to their wavelength.

32
Q

What does diffracting white light produce?

A

A spectra of colours.

33
Q

White light is a ______ of different ______, each with different ______.

A

White light is a (mixture) of different (colours), each with different (wavelengths).

34
Q

What happens when you shine a white light through a single narrow slit?

A

All of the different wavelengths are diffracted by different amounts, and so you get a spectra of colours.

35
Q

In diffraction patterns, what are the areas of light and dark called?

A

Dark and bright fringes.

The central bright fringe is called the central maximum.

36
Q

What precautions should you take whilst using a laser?

A

Whilst using a laser, you should:

  1. Never shine the laser towards a person
  2. Wear laser safety goggles
  3. Avoid shining the laser beam at a reflective surface.
  4. Have a warning sign on display
  5. Turn of the laser when it’s not needed
37
Q

Are interference patterns more or less defined/sharp when you use a higher diffraction grating?

A

Interference patterns are more defined/sharp when you use a higher diffraction grating because there are so many beams reinforcing the pattern.

38
Q

What is the line of maximum brightness in diffraction patterns referred to as?

A

The line of maximum brightness in diffraction patterns is referred to as the ‘zero order line’.

The lines either side of the zero order line are the first order lines and so on.

39
Q

What conclusions can you draw from dsinθ = nλ?

A

dsinθ = nλ

  1. If λ is bigger, sinθ is bigger, so θ is bigger. This means that the larger the wavelength, the more spread out the pattern will be.
  2. If d is bigger, sinθ is smaller. This means that the less lines per mm of a grating, the less spread out the diffraction pattern will be.
  3. If for a certain value of n you get a value for more than 1 for sinθ then you know that order doesn’t exist.
40
Q

Why does light slow down when it enters materials?

A

Light slows down in materials because it interacts with the particles in them.

The more optially dense a material is, the more light slows down when it enters it.

41
Q

What is the absolute refractive index of a material?

A

The absolute refractive index of a material is a measure of optical density.

It is found from the ratio between the speed of light in a vacuum, c, and the speed of light in that material, cs.

n = c/cs

42
Q

What is Snell’s Law?

A

Snells Law:

n1sinθ1 = n2sinθ2

43
Q

How do step-index optical fibres work?

A

Step-index optical fibres themselves have a high refractive index but are surrounded by cladding with a lower refractive index to allow total internal reflection.

Cladding also protects the fibre from scratches which could let light escape.

44
Q

What can cause a signal travelling through an optical fibre to degrade?

A

A signal (a stream of pulses of light) travelling down an optical fibre can be degraded by absorption or by dispersion.

Signal degradation can cause information to be lost.

45
Q

How does absorption of a signal travelling through an optical fibre degrade the signal?

A

As the signal travels, some of its energy is lost through absorption by the material the fibre is made from.

This energy loss results in the amplitude of the signal being reduced.

46
Q

How does modal dispersion of a signal travelling through an optical fibre degrade the signal?

A

The rays which take a longer path take longer to reach the other end than those that travel down the middle of the fibre.

47
Q

How can you stop modal dispersion from occuring in an optical fibre?

A

A single-mode fibre only lets light take one path, so it stops modal dispersion.

48
Q

How does material dispersion of a signal travelling through an optical fibre degrade the signal?

A

Material dispersion - light consists of different wavelengths that travel at different speeds in the fibre.

This causes some light wavelengths to reach the end of the fibre faster than others.

49
Q

How can you stop material dispersion from occuring in an optical fibre?

A

Using monochromatic light can stop material dispersion.

50
Q

What does dispersion cause in optical fibres?

A

Both types of dispersion (modal and material) lead to pulse broadening.

The signal sent down the fibre is broader at the other end.

Broadened pulses can overlap each other and confuse the signal.

51
Q

What can be used to reduce signal degradation in optical fibres?

A

An optical fibre repeater can be used to boost and regenerate the signal every so often, which can reduce signal degradation cause by both absorption and dispersion.

52
Q

Define coherence.

A

Coherent waves have a fixed phase difference and the same frequency and wavelength.

FIXED PHASE DIFFERENCE, SAME F & λ

53
Q

Why is a laser useful in showing interference and diffraction?

A

Lasers produce monochromatic light (same λ) so diffraction and interference patterns are more defined.

54
Q

What was Young’s double slit-experiment?

A
  • Single light source is directed towards two slits
  • These act as coherent light sources
  • Light interferes constructively and deconstructively to produce a diffraction pattern
55
Q

What is the order of colours when you direct a white light at a single slit to produce a diffraction pattern?

A

Diffraction pattern using white light:

  • Central white maximum at the zero order
  • Flanked by two spectral fringes, violet closest to 0 order and red furthest
56
Q

Why is a diffraction pattern formed when a light source is passed through a single slit?

A
  • The light diffracts as it passes through the slit
  • Where the waves are in phase, constructive interference occurs, making bright fringes
  • Where the waves are completely out of phase, deconstructive interference occurs, making dark fringes.
57
Q

What is the equation for fringe spacing?

A

Fringe Spacing

w = λD / s

w = fringe spacing
λ = wavelength
D = distance from slits to screen
s = spacing between slits
58
Q

What is modal dispersion?

A

Modal Dispersion: Light rays enter the fibre at different angles, and so take different paths.

59
Q

How can you prevent modal dispersion?

A

A single-mode fibre only lets light take one path, so it stops modal dispersion.

60
Q

When does total internal reflection occur?

A

Total internal reflection occurs when:

• When light is at a boundary to a less optically dense medium
AND
• Angle of incidence is greater than the critical angle

61
Q

How can you stop material dispersion?

A

Using monochromatic light can stop material dispersion.

Same λ, and so travel at the same speed.

62
Q

What is the equation for diffraction grating?

A

dsinθ = nλ

d = diffraction grating
sinθ = angle between 0 order and nth order
n = order maximum
λ = wavelength
63
Q

What is pulse broadening?

A

Pulse broadening

  • When the recieved signal is wider than the original.
  • Can cause the overlap of signals leading to information loss.
64
Q

What path does a light ray take when the angle of incidence is equal to the critical angle?

A

It goes along the boundary.

I.e., the angle of refraction is 90°.

AQA A-Level Physics (14 decks)

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