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Henry Physics RSFC > Waves 1 - Progressive waves and Refraction > Flashcards

Waves 1 - Progressive waves and Refraction Flashcards

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1
Q

What is a progressive wave?

A

Oscillations that have a resultant transfer of energy in one direction

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2
Q

How are mechanical and electromagnetic waves different?

A

Mechanical waves require a medium to oscillate through

Electromagnetic waves don’t require matter (oscillate through electric and magnetic fields)

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3
Q

What makes a wave transverse?

A

Oscillations are perpendicular to the transfer of energy

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4
Q

What makes a wave longitudinal?

A

Oscillations are parallel to the transfer of energy

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5
Q

What 2 properties do all electromagnetic waves possess?

A
  1. Always transverse
  2. Propagate with velocity of 3×108ms-1 through vacuum
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6
Q

Name 3 longitudinal waves

A
  • Sound
  • P-waves (Earthquakes)
  • Water waves (beneath surface)
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7
Q

Name 3 transverse waves

A
  • E-M waves (Light, X-rays, UV etc)
  • Waves on string
  • S-waves (Earthquakes)
  • Water waves (surface)
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8
Q

List in order all waves on the E-M spectrum

A
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9
Q

How are displacement and amplitude of a wave different?

A

Displacement → Current distance of a point from the equilibrium position

Amplitude → Maximum distance a point reaches from equilibrium position

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10
Q

Why do all points on a progressive wave have the same amplitude?

A

All points have the same maximum displacement from equilibrium position

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11
Q

What is the time period of a wave?

A

Time taken for each particle to complete one full oscillation

(Return to same position)

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12
Q

How is frequency of a wave defined?

A

The number of complete oscillations per second

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13
Q

What is the wavelength of a wave?

A

Distance between two adjacent corresponding points on a wave

(Same displacement, no phase difference)

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14
Q

What is the phase difference between A and B on this progressive wave?

A

360° ∼ 0°

2π∼ 0π

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15
Q

What is the phase difference between A and B on this progressive wave?

A

180°

π ∼ Antiphase

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16
Q

What is the phase difference between points A and B on this progressive wave?

A

540° ∼ 180°

3π ∼ π ∼ antiphase

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17
Q

How is phase difference calculated in degrees?

A
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18
Q

How is phase difference calculated in radians?

A
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19
Q

How do you convert from degrees → radians?

A
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20
Q

What is the phase difference between A and B on this progressive wave?

A

420° ∼ 60°

14π/6 ∼ π/3

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21
Q

How are frequency and wavelength related?

A
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22
Q

What are the 2 key features of longitudinal waves?

A

Compressions and rarefactions

23
Q

Why can’t sound waves be polarised?

A

Only transverse waves can be polarised

(Sound is longitudinal)

24
Q

What is the final intensity?

A
  1. Light vertically polarised through first grating
  2. Vertically p[olarised light can’t pass through horizontal grating
  3. Final intensity = 0
25
Q

What is the final intensity?

A
  1. Light vertically polarised through first grating (intensity halves)
  2. Vertically polarised light passes through second grating
  3. Final intensity = ½
26
Q

How do sunglasses reduce glare?

A
  1. When sunlight reflects off surfaces it is polarised
  2. Sunglasses have filter to block polarised light
  3. Only unpolarised light passes through
27
Q

What is the refractive index of a material?

A

Ratio of speed of light in a vacuum : speed light passes through material

(The greater n > the more light slows down)

28
Q

How does θ2 compare to θ1?

A

θ2 > θ1

(Light speeds up and bends away from normal)

29
Q

How does θ2 compare to θ1?

A

θ2 < θ1

(Light slows down and bends towards normal)

30
Q

Is the light refracting here?

A

Yes

It hasn’t bent towards or away from normal

But it has slowed down

31
Q

How does refraction affect the frequency of a wave?

A

Frequency does not change

(But wavespeed and wavelength do)

32
Q

What is dispersion?

A

Different wavelength refract by different amounts

So light passing through a prism separates into wavelengths

33
Q

What is wrong here?

A

In Snell’s law θ1 is the angle between normal and incident ray

34
Q

What are the 2 conditions for total internal reflection?

A
  1. θ1 > θc
  2. n2 < n1
35
Q

How is the critical angle calculated?

A
36
Q

How do you calculate the angle of incidence in the fibre?

A

Using basic geometry (angles in triangle add to 180°)

37
Q

Why are optical fibres better than copper cables?

A
  1. Information transmission faster
  2. More information can be transmitted (harder to intercept signals)
  3. Less energy loss (copper heats up)
38
Q

In optical fibres what does cladding do?

A
  1. Protects the core from scratches and spills
  2. Stops data loss to adjacent fibres
  3. Increases critical angle (reducing modal dispersion)
39
Q

What is modal dispersion and how is it combatted?

A

Different modes (angles) take different amount of time to propagate through an optical fibre

Leads to pulse broadening

Combatted by making core narrow and using cladding with low n (increasing θc)

40
Q

What is spectral (material) dispersion and how is it combatted?

A

Different wavelengths (colours) of light refracted by different amounts

Leads to pulse broadening

Combatted using monochromatic light

41
Q

What is oscillating in the case of a mechanical wave?

A

Particles are oscillating about their equilibrium position

42
Q

What is oscillating in the case of an electromagnetic wave?

A

Electric and magnetic fields

No particles are involved in oscillations

This is why electromagnetic waves can travel through a vacuum

43
Q

What do waves not transfer?

A

They DO NOT transfer matter

44
Q

Are all longitudinal waves mechanical?

A

Yes

EM waves transfer energy perpendicular to oscillations and so cannot be longitudinal

Therefore, any longtudinal wave can only be mechanical

45
Q

Are all mechanical waves longitudinal?

A

No

Mechanical waves can be longitudinal (sound waves) but can also be transverse (string)

46
Q

What occurs when the gradient of a displacement-distance graph for a longitudinal wave is:

  • Positive
  • Negative
A
  • When the gradient is positive, rarefactions occur
  • When the gradient is negative, compressions occur
47
Q

What is partial polarisation and when does it occur?

A

Where more light is transmitted in one plane than in other planes

This occurs during reflection, such as reflection off water, glass or metal

48
Q

How is light intensity emitted related to the angle of the second filter?

A

Intensity follows a cosine graph where initial intensity is max

If initial intensity is 0 it will follow a sine graph

49
Q

How does refractive index affect the speed of light?

A

Inversely

Higher refractive index → Slower speed of light

Lower refractive index → Higher speed of light

50
Q

Describe the process of reaching the critical angle

A
  • A ray of light passing from higher to a lower refractive index will refract away from the normal when the angle of incidence < critical angle and there will be a partially reflected ray
  • As the angle of incidence increases, the angle of refraction increases
  • The intensity of the reflected ray increases as the intensity of the refracted ray decreases
  • When the angle of incidence = critical angle, the angle of refraction is 90 degrees and travels along the boundary. There is still a reflected ray
  • If the angle of incidence increases further, (angle of incidence > critical angle), total internal reflection occurs. There is no refracted ray.
51
Q

What is attenuation and how is it overcome?

A

The decrease in the intensity of a wave over long distances

Solutions:

  • Using very high purity glass
  • This keeps the intensity as high as possible along the fibre
52
Q

Which end of the visible light spectrum has the highest refractive index?

A

Violet light
(shortest wavelength)
(slows down the most in refraction)

53
Q

Which end of the visible light spectrum has the highest refractive index?

A

Violet light
(shortest wavelength)
(slows down the most in refraction)

Henry Physics RSFC (27 decks)

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