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AQA A-Level Physics > Waves 2 > Flashcards

Waves 2 Flashcards

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1
Q

Define refraction

A

when a wave changes direction and speed as it enters a medium with a different optical density

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2
Q

When a wave is refracted, what else also happens?

A

the ray is partially refected

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3
Q

What do wavefronts join?

A

wavefronts join points of equal phase

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4
Q

What is the relationship between rays and wavefronts?

A

rays are perpendicular to wavefronts

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5
Q

What is Snell’s law?

A

n1(sin(i)) = n2(sin(r))

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6
Q

Define refractive index

A

the ratio of the speed of light in a vacuum to its speed in a given medium

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7
Q

What is the range for the refractive index always?

A

refractive index is always greater than or equal to 1

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8
Q

What refracts more, red light or blue light? Why?

A
  • blue light
  • the shorter the wavelength, the greater the angle of refraction
  • blue light travels slower in glass than red light, so bends more
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9
Q

What are the conditions for total internal reflection to occur?

A

1) the angle of incidence must be greater than the critical angle
2) the ray must be travelling in the optically denser medium

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10
Q

Define the critical angle

A

the angle of incidence where the angle of refraction equal 90°

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11
Q

What is the refractive index of air?

A

≈1

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12
Q

What is an optical fibre?

A

a narrow tube of glass that can carry light signals using total internal reflection

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13
Q

What are the three purposes of the cladding?

A

1) protects the core from scratching
2) stops two cores coming into contact and the light travelling between them (prevents data corruption and theft)
3) increases the critical angle to reduce the number of possible pathways for the light

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14
Q

What is pulse broadening?

A

when signal in an optical fibre gets wider and it transmits, due to dispersion

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15
Q

Why is pulse broadening problematic?

A

can cause adjacent bits to overlap and become indistinguishable

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16
Q

What are the two main reasons for pulse broadening?

A

1) modal dispersion

2) material dispersion

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17
Q

What is modal dispersion?

A
  • where light follows multiple paths of differing lengths

- light following the longer paths arrives after light that had taken a more direct route

18
Q

What is a solution to modal dispersion

A
  • use a fibre with a narrow core

the cladding also helps as a large critical angle limits the number of paths that propagate

19
Q

What is material dispersion?

A
  • dispersion that happens because blue light travels slower in glass compared to red light
  • the longer wavelengths in a signal therefore arrive before the shorter wavelengths, spreading the signal out
20
Q

What is the solution to material dispersion?

A

use monochromatic (single wavelength) light

21
Q

Give three advantages of optical fibres compared to copper cable

A
  • able to carry more simultaneous signals
  • faster data transmission
  • more secure
  • less susceptible to noise
  • needs boosting less frequently
22
Q

Give two disadvantages of optical fibres compared to copper cable

A
  • more difficult to modify
  • more expensive (to manufacture and install)
  • more complicated electronics required for interface and operation
23
Q

What is the wavelength of red light?

A

around 700nm

24
Q

What is the wavelength of blue light?

A

around 450nm

25
Q

Define diffraction

A

the spreading out of a wavefront after passing through a gap or around objects

26
Q

When is diffraction maximised?

A

when the gap width/obstacle is equal to the wavelength

27
Q

Describe the single slit diffraction pattern?

A
  • wide, bright, central maximum
  • subsequent maxima are half the width and progressively dimmer
  • fringes appear perpendicular to the slit
  • bright and dark regions (maxima and minima) alternate
28
Q

What happens to the single slit diffraction pattern if the slit becomes narrower?

A

there is a wider, dimmer central maximum

29
Q

What is the difference in the single slit diffraction pattern of blue light compared to red light? Why?

A
  • blue light has a smaller wavelength, therefore less diffraction occurs
  • so a narrower, brighter central maximum
30
Q

What is meant by w (fringe width)?

A

the bright and the dark region

31
Q

How can you reduce the percentage error when measuring w?

A

measure as many as possible and then divide

32
Q

What is meant for two sources to be coherent?

A

maintain a constant phase difference

33
Q

What is meant by monochromatic light?

A

light all of the same wavelength

34
Q

State the diffraction pattern from Young’s double slit experiment

A

equally spaced dark and bright fringes of equal intensity

35
Q

State the diffraction pattern from Young’s double slit experiment when using white light

A
  • a white central fringe with red edges
  • the other maxima would be spectrum
  • with blue closest to the centre and red furthest away 𝑤 = (𝜆D)/𝑠
36
Q

Explain the pattern from Young’s double slit experiment

A
  • bright fringes are formed when light constructively interferes
  • dark fringes are formed when light destructively interferes
37
Q

What is a diffraction grating?

A

a glass slide which contains lots of equally spaced vertical slits very close together

38
Q

What does d equal in the equation

d sin θ = nλ?

A

d equals slit spacing

39
Q

How can you find d from the number of slit per metre?

A

d = 1 / number of slits per m

40
Q

What is the diffraction pattern of white light though a diffraction grating?

A
  • n = 0 is a white light with red edges
  • n = 1 is a spectrum from blue (closest to the centre) to red
  • so on for n = 2, n=3, …
41
Q

What is the pattern from a diffraction grating?

A

sharp equally spaced bright fringes of equal intensity

AQA A-Level Physics (21 decks)

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