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3.3 Waves Flashcards

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1
Q

What is a progressive wave?

A

A transfer of energy from one place to another without transferring any material.

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2
Q

What is a longitudinal wave?

A

Oscillations are parallel to direction of energy transfer.

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3
Q

What is a transverse wave?

A

Oscillations are perpendicular to direction of energy transfer.

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4
Q

What is displacement? (Waves)

A

Distance moved from the equilibrium position on a point on a wave.

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5
Q

What is a cycle?

A

One complete oscillation of the wave.

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6
Q

What is amplitude?

A

Maximum magnitude of displacement from equilibrium position.

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7
Q

What is wavelength?

A

Length of one whole cycle between neighbouring identical points (i.e. crest to crest) with the same phase, on a wave.

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8
Q

What is a time period?

A

The time for one full cycle to complete or pass a given point.

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9
Q

What is frequency?

A

The number of cycles per second passing a given point.

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10
Q

What is phase difference?

A

A measure of how many degrees or radians two points on the same wave are from one another. It can also be measured in fractions of a cycle.

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11
Q

What is the speed of a wave?

A

Distance travelled by a wave per unit time.

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12
Q

What is the range of visible light?

A

Less than 400nm = UV.
400nm = violet/blue.
700nm = red.
More than 700nm = Infra-red.

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13
Q

What is absolute refractive index

A

Measure of the optical density equal to the ratio of the speed of light in a vacuum to the speed of light in a material.

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14
Q

What is relative refractive index?

A

Equal to the ratio of the speed of light in material 1 to the speed of light in material 2.

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15
Q

What is the critical angle?

A

Angle at which the angle of refraction equals 90˚.

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16
Q

What is total internal refraction?

A

When the angle of refraction is greater than 90˚ and all light is reflected back into the material.

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17
Q

What is absorption?

A

Where the energy of a wave is reduced through interaction with the material it is travelling through. Amplitude of signal reduced.

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18
Q

What is polarisation?

A

Only transverse waves can be polarised. (Also, light partially polarises on reflection).

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19
Q

What is the principle of superposition?

A

When two waves meet at a point, the resultant displacement is the vector sum of the individual displacements.

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20
Q

What is interference?

A

When two waves meet and superpose; the resultant displacement is the vector sum of the individual displacements.

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21
Q

What are 2 coherent waves?

A

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

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22
Q

What is path difference?

A

The extra number of wavelengths a wave must travel from one source than the other.

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23
Q

What is constructive interference?

A

When two waves meet and superpose, their resultant displacement is larger than their individual displacements
Occurs when path difference = nλ

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24
Q

What is destructive interference

A

When two waves meet and superpose, their resultant displacement is smaller than their individual displacements
Occurs when path difference = (n+1/2)λ (antiphase)

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25
Q

What is a stationary wave?

A

The superposition of two progressive waves, with the same frequency (wavelength), moving in opposite directions.
No energy is transferred by a stationary wave.

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26
Q

What is diffraction?

A

When a wave spreads out after passing through a gap or around an obstacle.

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27
Q

What is intensity?

A

Energy per unit area per unit time.

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28
Q

What is a node?

A

A point on a stationary wave of no displacement, zero amplitude.

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29
Q

What is an anti node?

A

A point on a stationary wave where the particles vibrate with maximum amplitude.

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30
Q

What is the fundamental node of vibration?

A

The lowest frequency wave that produces a stationary wave in a particular system.

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31
Q

What is a photon?

A

Quantum of energy of electromagnetic radiation.

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32
Q

What is an electron volt?

A

Energy acquired by an electron that is accelerated through a potential difference of 1 volt. 1eV= 1.6x10-19 J

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33
Q

What is work function?

A

The minimum energy required to release an electron from the surface of the metal.

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34
Q

What is threshold frequency?

A

Minimum frequency a photon must have to free an electron from the surface of a metal. (work function/planck constant).

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35
Q

What is monochromatic light?

A

Light of a single wavelength.

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36
Q

What are some examples of transverse waves?

A

EM waves
Waves on a string

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37
Q

What are some examples of longitudinal waves?

A

Sound

38
Q

What is the speed of EM waves in a vacuum?

A

They all travel at the same speed, c.

39
Q

What evidence do we have for the existence of transverse waves?

A

Scientists discovered that light was polarised by reflection. So they suggested that light was a transverse wave consisting of vibrating electric and magnetic waves at right angles. Which was why light could be polarised.

40
Q

What are the uses of polarising filters?

A

They are used in sunglasses as if you view reflected partially polarised light through a polarising filter at the correct angle, you can block out unwanted glare. (The unwanted glare can be created by light reflected by a transparent surface like light.) Car windows also have the same effect.

41
Q

What are the uses of Polarisation in TV?

A

TV signals are polarised. TV aerials on peoples houses have horizontal rods. The signal is polarised by the orientation of the rods on the broadcasting aerials. For a strong signal, the rods on the receiving aerial are lined up with the rods on the transmitting aerial.

42
Q

What are the uses of polarisation in photography?

A

It alters the appearance of the sky caused by particles in the atmosphere which create partial polarisation by scattering sunlight.

43
Q

What happens to a light wave when it hits a transparent object such as glass or water?

A

The reflected ray is partially polarised.

44
Q

How can visible light be polarised?

A

A polarising filter
Reflection
Scattering

45
Q

What is vertically polarised?

A

All oscillations in one plane.

46
Q

How can polarisation be used for stress and strain analysis?

A

For plastics and perspects, polarised light micrography is a useful technique in the analysis of crystal structure.

47
Q

What are the bright fringes in the double slit diffraction pattern?

A

Areas of constructive interference.

48
Q

How can you conduct Young’s double slit experiment?

A

You can use 2 separate, coherent light sources or you can shine a laser through 2 slits. Laser light is coherent and monochromatic.
Shine the laser through 2 slits onto a screen.
The slits have to be about the same size as the wavelength of the laser so that it is diffracted.
You get a pattern of light and dark fringes.
Make sure distance between sources and screen is greater than 2m and measure 5 or more fringe spacings with a vernier calliper to reduce percentage uncertainty.

49
Q

What safety precautions must be taken when using a laser?

A

Don’t look directly into the beam.
Don’t shine on a reflective surface.
Avoid shining on a person.
Wear laser safety goggles.
Display a warning sign.
Turn the laser off when it’s not needed.

50
Q

How can you produce monochromatic light?

A

A laser
A coloured filter between a light source and screen. (Image may be dim as most of the light is absorbed.)

51
Q

How can you produce a stationary wave?

A

Set up a driving oscillator at one end of a stretched string with another fixed end. The wave generated by the oscillator in reflected back and forth. If the oscillator happens to produce an exact number of waves, the original and reflected wave reinforce each other.

52
Q

Which factors affect the frequency of a standing wave?

A

Length- increasing length decreases frequency
Tension- increasing tension increases frequency
Mass- increasing mass decreases frequency

53
Q

How do you produce coherent light from white light?

A

Use a single slit or use a monochromatic light source instead.

54
Q

What are the characteristics of the interference pattern created with white light in the double slit?

A

Central white fringe
• Other fringes contain colours
• Maxima become wider (moving from the centre)
• Minima become narrower (moving from the centre)
• Maxima are more intense (than monochromatic light)

55
Q

Which factors affect fringe separation?

A

Distance- increasing distance increases fringe spacing
Wavelength- increasing wavelength increases fringe spacing
Slit spacing- increasing slit spacing decreases fringe spacing

56
Q

What are the characteristics of the interference pattern created when monochromatic light is diffracted through a single slit?

A

If the wavelength is the same size as the aperture:
There’s a central bright fringe
The central fringe is twice as wide

57
Q

What happens when white light is diffracted through a single slit?

A

All of the different wavelengths are diffracted by different amounts. So, instead of getting clear fringes, you get a spectra of colours going from blue to red as you go outwards.

58
Q

What are the characteristics of the interference pattern created by white light through a single slit?

A

Bright, wide central white fringe
Subsidiary maxima look like spectra
Moving out from the centre, colours go from violet to red
Maxima are wider and minima are smaller compared to monochromatic light.

59
Q

When light is shone through a single slit, which 2 things affect the width of the central maximum?

A

Increasing slit width decreases diffraction so, the central maximum is narrower and the intensity of the central maximum is higher.
Increasing the wavelength increases diffraction. So, the central maximum is wider and the intensity is lower.

60
Q

What happens when the gap through which the waves travel has a much smaller width than the wavelength?

A

The waves are mostly reflected back.

61
Q

How can you demonstrate Young’s double slit experiment with water waves?

A

It is easy to measure as the wavelengths are a good size. You need 2 coherent sources so, you need to use the same vibrator to drive both sources. So, one vibrator drives 2 dippers.

62
Q

How can you demonstrate Young’s double slit experiment for sound?

A

It is easy to measure as the wavelengths are a good size. You need 2 coherent sources so, you need to use the same vibrator to drive both sources. So, one oscillator is connected to 2 loud speakers.

63
Q

How can you conduct the double slit experiment with microwaves?

A

Replace the laser and slits with 2 microwave transmitter cones attached to the same signal generator.
Replace the screen with a microwave receiver probe.
Move the probe along the path of the green arrow, you’ll get an alternating pattern of strong and weak signals.

64
Q

Why is the central maximum in a single slit diffraction the brightest part?

A

The intensity is highest in the centre as there is an increase in photons per second.

65
Q

Which theory did Youngs double slit experiment help to support?

A

The wave nature of EM radiation. Newton’s corpuscular theory explained reflection and refraction but not diffraction and interference. It provided evidence towards Huygen’s theory of waves.

66
Q

What is the difference in the interference pattern between double slit and diffraction grating?

A

The shape of the pattern is the same. But, for the diffraction grating, the bright bands are brighter and narrower and the dark bands are darker. When monochromatic light is passed through a diffraction grating, the pattern is really sharp as there are many slits reinforcing the pattern. Sharper fringes means more accurate measurements.

67
Q

What happens to the interference pattern from the diffraction grating when wavelength is increased?

A

Sin(theta) is bigger so, theta increases. So, the pattern will spread out.

68
Q

What happens to the interference pattern from the diffraction grating when slit spacing is increased?

A

Sin(theta) is smaller. So, the coarser the grating, the less the pattern will spread out.

69
Q

What are the uses of diffraction gratings?

A

Identifying elements (chemical analysis)
X-ray crystallography
Studying Doppler shift of stars
Spectral analysis of stars to study their composition

70
Q

How are diffraction gratings used in identifying elements?

A

White light is a mixture of colours. So, when it’s diffracted, the light spreads out by different amounts so each order becomes a spectrum with red on the outside and violet on the outside. The zero order maximum stays white as light goes straight through. Astronomers and chemists study spectra to identify elements as they’re more accurate than using prisms.

71
Q

How are diffraction gratings used in X-ray crystallography?

A

The wavelength of X-rays is a similar scale to the spacing between atoms in crystalline solids. So, X-rays will form a diffraction pattern when directed at a thin crystal. The crystal acts as a diffraction grating and the spacing between the atoms, the slit width, can be found from the diffraction pattern.

72
Q

What is Snell’s law?

A

The angle the incoming light makes with the normal is called the angle of incidence. The angle the refracted ray makes with the normal is the angle of refraction. nsin(theta) = n2sin(theta2)

73
Q

What happens when light enters an optically denser medium?

A

Light is refracted towards normal.

74
Q

What is one use of total internal refraction?

A

Optical fibres

75
Q

How do optical fibres use total internal refraction?

A

An optical fibre is a very thin flexible tube of glass or plastic fibre that carries light signals over long distances and around corners. Step- index optical fibres have a high refractive index but are surrounded by cladding with a low refractive index to allow total internal refraction. The cladding protects te fibre from scratches which might let light escape. Light is shone from one end. The fibre is narrow so that the light hits the boundary at an angle greater than the critical angle.

76
Q

How is signal degradation caused?

A

Absorption
Dispersion
This may cause some of the signal to be lost.

77
Q

What are the effects of absorption of the signal?

A

Some of the energy is lost by the material the fibre is made from. The energy loss results in a reduced amplitude.

78
Q

What are the 2 types of dispersion?

A

Modal
Material

79
Q

What is modal dispersion?

A

When light rays enter the fibre at different angles. So, the light takes different paths. The rays which take a longer path take longer to reach the other end than those which travel down the middle of the fibre.

80
Q

How is modal dispersion prevented?

A

A single-mode fibre only allows the light to take one path, stopping modal dispersion.

81
Q

What is material dispersion?

A

When light consists of different wavelengths that travel at different speeds in the fibre. So, some wavelengths reach the end of the fibre faster than others.

82
Q

How is material dispersion prevented?

A

Use of monochromatic light.

83
Q

What is the effect of dispersion on the signal?

A

Pulse-broadening. The signal sent down the fibre is broader at the other end. The broadened pulses overlap each other and confuse the signal.

84
Q

How is signal degradation overall reduced?

A

An optical fibre repeater can be used to boost and regenerate the signal every so often.

85
Q

What are the characteristics of the interference pattern created by white light through a diffraction grating?

A

The central maximum is narrower than single slit diffraction
The central maximum is white and thin
The subsidiary maxima are spectra
The spectra become wider as the order increases.

86
Q

What is the refractive index of air?

A

Approximately 1

87
Q

When wind blows, stationary waves can be formed on the aerial wire.

Explain how stationary waves are produced and why only waves of specific frequencies can form on the aerial wire.
(4 marks)

A

Wind produces a wave / disturbance that travels along the wire.
Wave is reflected at each end / waves travel in opposite directions.
(Incident and reflected) waves interfere / superpose.
Only certain frequencies since fixed ends have to be nodes.

88
Q

Describe the pattern produced on screen when white light is shone through a slit with a green filter infront.
(2 marks)

A

Central maximum with lower intensity maxima (either side)

Central maximum is twice as wide/wider than other maxima.

89
Q

White light is shone through a slit with a green filter infront. Describe the change in the pattern produced on screen when the filter is replaced with a red filter.
(2 marks)

A

Wider (central) maxima (maximum)

(Subsequent) maxima further apart

90
Q

A diffraction grating is placed between the red filter and the screen. The filter transmits red light with wavelengths in the range 600 nm to 700 nm, and not just light of the wavelength 650nm.
Suggest how this affects the appearance of the maxima.
(2 marks)

A

Central maximum unchanged in width
Broader maxima/range of angles for each maximum/order
Gradually getting broader/more spread out for greater order maxima

91
Q

The figure shows an optical fibre. Where X is the centre and Y is the outer.
Discuss the properties of a step-index optical fibre.
Your answer should include:
•   the names of part X and part Y
•   a description of the functions of X and Y
•   a discussion of the problems caused by material dispersion and modal dispersion and how these problems can be overcome.

A

X is Core
Y is Cladding Functions
X: Propagates light by TIR (with) low attenuation/absorption
Refractive index of core > cladding
Y: Protects core from damage, Prevents cross talk between touching fibres, Provides ‘clean’ boundary for TIR
Dispersion problems
Both: Cause pulse broadening
Material: different wavelengths have different speeds due to different refractive indices within the core — use monochromatic beam
Modal: different paths have different lengths so effective time along fibre differs - use single-mode fibre

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