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X AS Level Physics > waves > Flashcards

waves Flashcards

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1
Q

what is a progressive wave?

A

a wave that carries energy from one place to another without transferring material.

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2
Q

How can you tell certain types of waves carry energy?

A

The source will lose energy. X-ray and gamma rays knock electrons out of their orbits, causing ionization.

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3
Q

What is the displacement of a wave?

A

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

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4
Q

What is the amplitude of a wave?

A

The maximum displacement.

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5
Q

What is the wavelength of a wave?

A

the length of one whole wave from crest to crest or from trough to trough. The shortest distance between 2 adjacent points in phase. Measured in meters.

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6
Q

What is the period of a wave?

A

The time taken for one whole vibration. Measured in seconds.

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7
Q

What is the frequency of a wave?

A

The no. of vibrations per second passing a given point. Measured in hertz.

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8
Q

What is the phase difference of a wave?

A

The amount which one wave lags behind another wave. Measured in degrees or radians.

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9
Q

Define reflection.

A

The wave is bounced back when it hits a boundary.

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10
Q

What is refraction?

A

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

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11
Q

How is the frequency of a wave found?

A

1 / period.

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12
Q

How can the speed of a wave be found?

A

speed= wavelength X frequency.

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13
Q

What is a transverse wave?

A

The vibrations are at right angles to the direction of travel.

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14
Q

What are the 2 ways transverse waves can be drawn on a graph?

A

Displacement against against distance along the path of the wave. Displacement against time.

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15
Q

What are longitudinal waves?

A

The vibrations are along the direction of travel. They consist of areas of compression and rarefaction in the medium they travel through.

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16
Q

What is a polarized wave?

A

A wave that only oscillates in one direction. It can only happen with transverse waves.

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17
Q

What does a polarizing filter do?

A

It will only let certain waves through that are at the right angle.

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18
Q

What would happen if you shone light through 2 polarizing filters at 90 to each other.

A

No light would get through.

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19
Q

How do em waves work?

A

They consist of vibrating electric and magnetic fields.

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20
Q

What is the order of the em spectrum?

A

radio, micro, infared, visible ultraviolet, x-rays gamma rays.

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21
Q

What is the relation to the energy of an em wave and where it comes from in the atom?

A

The greater the energy, the closer to the center of the atom it came. Gamma waves come from inside the nucleus, x-rays to visible light come from energy level transitions in atoms, Infared and microwave come from molecules. Radio waves come from oscillations in electric fields.

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22
Q

What is the optical density of a material?

A

The more optically dense a material is, the slower light will go when within it.

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23
Q

What is the absolute refractive index of a material?

A

The ratio of the speed of light in a vacuum and the speed of light in the material. Divide the speed of light by the speed of light in the material.

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24
Q

What is a relative refractive index?

A

The ratio between the speed of light in one material compared to another. Divide the speed of light in a vacuum by that of the material. It should be a number less than 1

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25
Q

What is the angle of incidence refraction?

A

The angle the incoming light makes will the normal and the angle the refracted light makes with the horizontal.

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26
Q

What happens when light goes from one material into another with a greater optical density?

A

It bends towards the normal.

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27
Q

What do you assume the speed of light in air to be?

A

The speed of light in a vacume.

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28
Q

What is the angle of refraction?

A

The angle the refracted Ray makes with the normal.

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29
Q

What does n mean?

A

The refractive index of a material.

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30
Q

What is smells law?

A

n₁sin i = n₂sin r

The 2 n’s being the refractive index of the material the light was going from and that of the one it is going into.

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31
Q

When do you need to know the refractive index of a material?

A

When making lenses. Different types of lenses have different refractive indexes meaning you have to know them to shape the lenses to get the desired affect.

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32
Q

What happens when a beam of light goes from an optically denser material to a less dence one?

A

It will be refracted away from the normal. As the angle of incidence is increased it goes further away from it. The angle of refraction gets closer to 90˚.

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33
Q

What is the critical angle?

A

The angle of incidence that will cause the angle of refraction to be 90˚
At this angle, the light will go along the boundary of the material.
n sin c = 1 where c is the critical angle.

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34
Q

What happens when the angle of incidence is greater than C?

A

Refraction is impossible and all the light will be reflected back into the material. This is total internal reflection.

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35
Q

How does an optical fibre work?

A

It is a thin flexible tube of glass or plastic that has a high refractive index which is enclosed in a cladding with a lower refractive index.
When light is shone through, the light will hit the boundary between fibre and cladding at an angle bigger than the critical angle because fibre is so thin.

36
Q

What is the advantage of optical fibre over electrical cables?

A

The signal can carry more information because the light has a larger frequency.
The light doesn’t heat up the fibre so almost no energy is lost.
There is no electrical interference.

37
Q

What is superposition ?

A

It happens when 2 or more waves meet each other. The resultant displacement equals the vector sum of the individual displacements.

38
Q

What are the types of interference ?

A

Constructive. This is when a crest and a crest or a trough and a trough meet to give bigger ones.
Destructive interference is when a crest and a trough of equal size meet to cancel out.

39
Q

What does it mean for 2 points on a wave to be in phase?

A

If they are on the same point in the wave cycle. There will have the same displacement and velocity.
2 points with a phase difference of zero or a multiple of 360 ˚ are in phase.
Points with a phase difference of odd multiples of 180˚ are exactly out of phase.

40
Q

When are 2 sources coherent?

A

If they have the same wavelength and frequency and a fixed phase difference between them.

41
Q

What is the path difference?

A

Imagine having 2 speakers one next to the other. The amount by which the path travelled by one wave is longer than the path travelled by the other wave is called the path difference.

42
Q

What is the path difference were there is constructive interference?

A

There can be no path difference or the path difference is a whole number of wavelengths. At these points the waves are in phase and will reinforce each other.

43
Q

Where does destructive interference happen in terms of path difference?

A

At points where the path difference is half a wavelength, 1.5 wavelengths 2.5 and so on.

44
Q

What is a standing wave?

A

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

45
Q

What type of wave doesn’t transfer energy?

A

Standing waves.

46
Q

What is a node?

A

Where the amplitude of the vibration is zero. Antipodes are points of maximum amplitude.

47
Q

What is a resonant frequency ?

A

Frequencies that mean an exact number of half wavelengths fits onto the string in a standing wave.

48
Q

What is the fundamental frequency?

A

The lowest possible resonant frequency with a standing wave.

It has one antinode and 2 nodes at each end.

49
Q

What is the second harmonic?

A

Twice the fundamental frequency. One whole frequency fits on the string with 2 antinodes and 3 nodes.

50
Q

What is the third harmonic?

A

3 times the fundamental frequency. 1.5 wavelengths 4 nodes, 3 antinodes.

51
Q

Do all waves diffract?

A

Yes

52
Q

When a wave goes through a gap a lot greater than wavelength what is the diffraction like?

A

The diffraction is unnoticeable.

53
Q

What happens when a wave goes through gap serval wavelengths wide?

A

There is noticeable diffraction at the sides.

54
Q

What happens when a wave goes through gap of width equal to that of the wavelength?

A

There is maximum diffraction.

55
Q

What happens when a wave goes through gap smaller than the wavelength?

A

Most of the wave will be reflected back and not diffracted.

56
Q

What happens when a wave meets an obstacle?

A

The wave will diffract around the edges. Behind the obstacle there will be a shadow where the wave is blocked.

57
Q

What happens when you diffract light through a gap the same size as the wavelength?

A

There is a bright central fringe with alternating dark and bright fringes on either side of it.

58
Q

Why can’t you arrange two separate coherent light sources?

A

Because light from both will be emitted randomly.

59
Q

What does monochromatic mean?

A

One frequency.

60
Q

In the fridge spacing formula, what does s mean?

A

The spacing between slits.

61
Q

In the fridge spacing formula, what does D mean?

A

The distance from slits to screen.

62
Q

What happens when you shine light through more than 2 slits?

A

You will still get bands of light and dark through the lighter parts are lighter and the darker parts are darker.

63
Q

When light is shone through a diffraction grating, what is the zero order line?

A

The brightest line that goes through the middle.

The beams of light either side of this are the first order lines, then second order lines and so on.

64
Q

What does d mean in the diffraction grating equation?

A

The distance between the slits in the grating.

65
Q

What is the angle in the diffraction grating equation?

A

The angle between the zero order line and a diffracted beam.

66
Q

When a beam of light is shone through a single slit, what are the observations?

A

A series of maxima and minima.
The pattern is symmetrical.
The central maximum is twice as wide as the others.
The intensity falls of as you get further from the central maximum.
The gap depends on wavelength slit width and distance from the screen.

67
Q

Why is a laser used when observing diffraction?

A

It is monochromatic and coherent

68
Q

What are the conditions needed for TIR?

A

Angle of incidence > critical angle.

Light goes from a greater refractive index to a less refractive index.

69
Q

What are the disadvantages of fibre optics?

A

Light can be absorbed or scattered
Intensity can fall.
Multipath dispersion. Light travelling along the axis travels a shorter path then light. Being reflected causes a broadening of the pulse.
Spectral dispersion. Different wavelengths travel at different speeds causing the shape of the signal to be elongated.

70
Q

How can the problems of fibre optics be reduced?

A

Use monochromatic light.
Use boosters to boost the signal.
Have a narrow core and a small difference in refractive indexs.
Use a wavelength that wouldn’t be absorbed. (Inferred)
Info tranmitted in digit form since minor degradation can be corrected without loss of data.

71
Q

Why in optic fibre, is a small core wanted?

A 
Less light is lost.
Better quality signal/less distortion
Increased chance of TIR.
Less change of angle between each reflection.
Keeps incident angle greater than CA
Less refraction out of core.
Faster data transfer.
Less multipath dispersion (overlapping)
72
Q

Why does optical fibre have a cladding?

A

Protect the core
Prevent loss of data.
Increase rate of data transfer.
Increase the critical angle

73
Q

When light goes from air to glass, what will the values in c=fλ change?

A

C slows down,
F no change
λ gets shorter.

74
Q

When when a wave is put through an optical fibre why does the amplitude fall?

A

Energy is lost or absorbed in the fibre.

75
Q

When when a wave is put through an optical fibre why does the width increase?

A

Pulse broadening caused by multi path dispersion. Different rays propagate from different angles. They take different times to travel.

76
Q

In the fringe spacing equation, what do the separate parts mean?

A

D is distance from the slits to the screen.
λ is the wavelength.
S is the difference between the gratings.
W is the fringe spacing in meters

77
Q

What are the conditions needed for total internal refraction?

A

Angle of incidence is greater than the critical angle.

n₁ > n₂. It goes from a more to a less optically dense material.

78
Q

What are phase differences measured in?

A

Always in matter of degrees.

79
Q

How can the phase difference between 2 points be found?

A

Distance between two points ÷ wavelength

Then multiply by 360

80
Q

Why when young did his light experiments, was the first slit used?

A

To provide coherent light.

It was narrow to ensure the 2 other slits were properly illuminated.

81
Q

Why when 2 slits are used in an experiment, do they produce light in phase?

A

The paths to the different slots should be the same length.

82
Q

When light is shone through 2 slits, what do you mention?

A

Light superposes
Interference patterns form.
Areas of light and dark

83
Q

When white light is shone through a slit, what do you see?

A

A white central fringe.
The fringes show a spectrum of colour. Blue towards the middle, red away.
Fringes wider then if one colour was used.

84
Q

What does the graph of light shone through a double slit look like?

A

All fringes have same width.
Central is not twice as bright.
Gets slightly less intense as it goes to the side.

85
Q

What happens if the slit, light is shone through is made thinner?

A

Reduced intensity.

The maxima are further apart.

86
Q

What happens if the light shone through a slit has the wavelength increased?

A

Maxima further apart.

X AS Level Physics (10 decks)

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