Waves Flashcards

1
Q

What is the equation for Snell’s Law?

A

n1/n2 = sinθ2 / sinθ1

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2
Q

What is an optical fibre?

A

A thin glass/plastic fibre that transmits light signals via total internal reflection

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3
Q

What is a step-index optical fibre?

A

An optical fibre with a uniform refractive index in the core and a smaller refractive index in the cladding

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4
Q

What is material dispersion?

A

*When white light is used instead of monochromatic light
* Diff wavelengths (from white) travel at different speeds
* Causing the light to spread out as it transmits
* Pulse broadening

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5
Q

What does multimode and monomode mean?

A

multimode = wide optical fibre core
monomode = narrow optical fibre core

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6
Q

What can be done to prevent material dispersion?

A

Use monochromatic light (single wavelength)

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7
Q

What is modal dispersion?

A

When beams of light enter the fibre at diff angles -> each beam undergoes TIR a diff no. of times -> each beam reaches the end at diff times -> pulse broadening

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8
Q

What is pulse broadening?

A

When the duration of a signal increases due to modal or material dispersion

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9
Q

What are the 2 conditions for total internal reflection?

A
  1. angle of incidence > critical angle
  2. n1 > n2
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10
Q

When the ray goes from a low to a high refractive index, light bends _______ the normal.

A

towards

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11
Q

When the ray goes from a high to a low refractive index, light bends _______ the normal.

A

away

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12
Q

When a pulse of light travels down an optical fibre the amplitude decreases. Why is this?

A

Energy from the signal is absorbed by the optical fibre

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13
Q

Why does light change direction when travelling from one substance to another?

A

Light has different speeds in different substances:
-when light travels towards a denser substance it slows down -> bends toward the normal

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14
Q

The only properties that change during diffraction are _______ and _______. ____________ does not change.

A

speed / wavelength / frequency

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15
Q

Why do the intensity and amplitude slightly decrease upon refraction?

A

A small portion of light also reflects

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16
Q

What is the refractive index (n)?

A

Shows you how much light slows down when passing through a material

17
Q

What is the critical angle?

A

The critical angle is the angle of incidence when the angle of refraction is exactly 90° and the light is refracted along the boundary

18
Q

What is the equation to find the critical angle?

A

sin θc = n2 / n1

19
Q

What are 2 uses of optical fibres

A
  • Communication: Internet transmission and telephone
  • Medical: endoscope
20
Q

How do optical fibres allow TIR to occur?

A

The refractive index of the core is more than the refractive index of the cladding
n cladding < n core

21
Q

Why is cladding required on optical fibres?

A
  • protects core from damage
  • prevents signal loss through light escaping the core
  • prevents information from transferring between different cores in a bundle
22
Q

What is phase difference?

A

How much a wave lags behind another wave

23
Q

What is phase difference?

24
Q

Why must the optical fibre core be protected from scratches?

A
  • water can get into scratch -> increase n to become higher than the core n -> prevent TIR -> signal lost

-scratch can alter angle signal where light TIR -> if angle lowered below critical angle -> prevents TIR -> signal lost

25
What is absorption?
where energy is lost as the signal is transferred -> loss of amplitude but doesn't affect frequency
26
What is absorption?
where energy is lost as the signal is transferred -> loss of amplitude but doesn't affect frequency
27
What does coherent mean?
The waves must have the same: -frequency -wavelength -have a fixed phase relation
28
What is path difference?
The difference in the length travelled between 2 waves to get to a certain point
29
How do you calculate path difference for 2 coherent wave sources?
(When path difference is a multiple of λ) path difference = nλ
30
How do you calculate path difference for waves that are exactly out of phase? (destructive)
(When path difference is whole no. + 1/2 λ) path difference = (n+1/2)λ
31
What did Young do to ensure the 2 waves were coherent in his experiment?
- it is tricky to ensure 2 sources are coherent - laser produces monochromatic coherent light - the 2 slits act as 2 identical sources of laser light - slits have to be the same size to have a comparable wavelength to diffract
32
What must you do to ensure that the fringe measurement is accurate?
Measure across lots of fringes and divide by the no. of fringes to get an average
33
What would we see if we did Young's double slit experiment with white light?
Instead of bright and dark fringes -central fringe is white light -all fringes are more spread out -side fringes have a spectrum of visible colours -blue diffracts less than red -> blue is nearer the centre