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Physics > 3.3 Waves > Flashcards

3.3 Waves Flashcards

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1
Q

Purpose of core in a step-index optical fibre

A

Core is transmission medium for electromagnetic waves to progress by total internal reflection (wit low absorption)

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2
Q

Define wavelength

A

Distance between two adjacent peaks on a wave

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3
Q

Define amplitude

A

The maximum displacement of the wave from its equilibrium position

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4
Q

What are areas of high and low pressure called in a longitudinal wave?

A

High -> compressions
Low -> rarefactions

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5
Q

What does a polarising filter do?

A

Only allows oscillations in one plane

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6
Q

How is polarisation used as evidence of the nature of transverse waves?

A

Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (as they are in transverse waves)

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7
Q

Describe the appearance of the first harmonic

A

Two nodes at either end and an antinode in the middle

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8
Q

What is meant by coherence?

A
  • Constant phase difference
  • and the same frequency and wavelength
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9
Q

When does most diffraction occur in single/two source intereference?

A
  • When slit is close to wavelength in size
  • so lots of diffraction occurs
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10
Q

What is phase?

A

The position of a certain point on a wave cycle

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11
Q

What is a progressive wave?

A

A wave that transfers energy without transferring material

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12
Q

4 safety measures when using lasers

A
  1. Never shine the laser towards a person
  2. Wear high opacity safety goggles
  3. Have a warning sign on display
  4. Turn the laser off when it is not needed
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13
Q

State the principle of superposition

A

When two or more waves cross, the resultant displacement of a particle is the sum of the displacements caused by the individual waves

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14
Q

Explain how double slit diffraction occurs

A
  • Intense light from a laser hits a pair of slits
  • Light diffracts from the two slits, spreading out in circles
  • The light from the two slits is coherent as it is derived from the same source
  • The light from both slits strikes a screen
  • When two waves cross, they superpose
  • The path difference from each slit to a point on the screen causes a phase difference
  • Phase difference of 0 = whole number of half wavelength causes constructive interference–> max amplitude and bright spot
  • and vice versa
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15
Q

What happens as slit width approaches the size of the wavelength?

A

Diffraction becomes more observable

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16
Q

Why does single slit diffraction occur?

A
  • Waves from different edges of the slit travel difference distances to the screen
  • So there is a path difference
  • A path difference means a phase difference
  • –> Constructive and destructive interference.
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17
Q

Describe the single slit pattern

A
  • Bright central fringe with fringes either side of decreasing intensity
  • The central fringe has twice the width of those either side
  • The central fringe has a much larger intensity than those either side
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18
Q

2 applications of diffraction gratings

A
  • Measuring the wavelength of light from stars (using absorption spectra) to determine the constituents in the star’s atmosphere
  • X-ray crystallography for finding the spacing between atoms - a crystal sheet acts as the diffraction grating for X-rays to pass through
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19
Q

Which colour makes the smallest angle of refraction? (think Pink Floyd)

A

Red

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20
Q

Why can chromatic dispersion be observed easily using a glass prism?

A

The light refracts twice, which makes the change in angle of refraction more obvious

21
Q

Why does modal dispersion occur?

A

Different rays enter the core at different angles, and so travel different paths (some may travel middle while others reflect repeatedly), so the time of arrival at the end of the fibre varies

22
Q

Why does material/modal dispersion mean less data being sent down the fibre? (3*)

A
  • Material/modal dispersion causes pulse broadening, which causes received signals to be wider than the original signal
  • Light pulses that are too close will become merged at the end of the fibre
  • So signals must be sent at longer time intervals
23
Q

Why does material dispersion occur?

A
  • Different wavelengths travel at different speeds in a fibre
  • Because refractive index depends of wavelength
24
Q

How do you fix material dispersion?

A

Using monochromatic light

25
Q

Why is there a reduced amplitude at the other end of an optical fibre?

A

Due to absorption of energy within the fibre

26
Q

Why is a smaller core better for fibre optics?

A
  • There is an increased probability of total internal reflection
  • So less refraction out of the core
  • So less light is lost
27
Q

How can modal dispersion be reduced?

A

Using a single-mode fibre, which only allows light to take a very narrow path, so the possible difference in path lengths is smaller

28
Q

For a sound wave, explain what is meant by wavelength

A
  • wavelength is shortest distance between two points in phase
  • so it is the distance between two adjacent compressions (or rarefactions)
29
Q

Explain how a bright line is formed by the diffraction grating at the first-order diffraction angle

A
  • light from each slit superpose
  • light from adjacent slits have a path difference of one wavelength
  • at this angle all the waves are in phase
  • constructive interference occurs
30
Q

What is a stationary wave? (3*)

A
  • A wave which does not transfer any energy
  • Its positions of maximum and minimum amplitude are constant (called anti-nodes and nodes respectively)
  • The energy is stored in the motion of the antinodes
31
Q

Describe briefly what Young’s double-slit experiment was

A
  • A single light source is directed towards to slits, which each act as a coherent light source
  • The light interferes constructively and destructively to create an interference pattern
32
Q

Explain the effect of increases the slit width on the central maximum in single slit diffraction?

A
  • The slit is not close to the wavelength in size, so less diffraction occurs
  • So the central maximum becomes narrower and more intense
33
Q

What is the purpose of the cladding in an optical fibre?

A
  • Protects core from scratches which would allow light to escape and degrade the signal
  • Allows total internal reflection as it has a lower refractive index than the core
  • Prevents cross-talk between touching wires
34
Q

How does signal degradation by absorption in an optical fibre affect the received signal?

A

Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced

35
Q

What is pulse broadening? Why is it a problem?

A
  • When the received signal is wider than the original
  • This can cause overlap of signals leading to information loss
36
Q

How can both absorption and dispersion be reduced in an optical fibre?

A

Use an optical fibre repeater to regenerate the signal every now and then

37
Q

4 advantages of optical fibres over traditional copper wires

A
  • Signals can carry more information as light has a high frequency
  • No energy lost as heat
  • No electrical interference
  • Very fast
38
Q

What is phase difference?

A

How much a particle/wave lags behind another particle/wave

39
Q

How are polarisers used in antennas?

A

The receiving aerial must be aligned in the same plane of polarisation as the transmitting aerials in order to receive the signal at full strength

40
Q

Why is a laser useful in showing interference and diffraction?

A
  • It produces monochromatic light, of one wavelength
  • So diffraction and interference patterns are more defined
41
Q

Describe the two-source interference pattern created using white light

A

A bright white central maximum flanked by alternating spectral fringes of decreasing intensity, with violet closest to the central maximum, and red furthest

42
Q

When does total internal reflection occur?

A
  • When light is at a boundary to a medium with a lower refractive index
  • And the angle of incidence is greater than the critical angle
43
Q

Which wavelengths diffract at greater angles?

A

Longer wavelengths (e.g. red)

44
Q

Which wavelengths diffract at smaller angles?

A

Shorter wavelengths (e.g. blue)

45
Q

How did Young’s double slit experiment provide evidence for the wave nature of light?

A
  • Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties
  • Particles would just produce a bright spot in the centre of the screen
46
Q

How could you investigate stationary sound waves?

A
  • Place a speaker at one end of a closed glass tube, lay power across the bottom of the tube
  • The powder will be shaken from the antinodes and settle at the nodes
  • The distance between each node is half a wavelength
47
Q

Conditions for a stationary wave to form

A
  • 2 waves travelling in opposite directions
  • Same frequency and wavelength (hence same speed)
  • Similar amplitudes
48
Q

What do you always draw for refraction?

A

The partial reflection

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