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Engineering Physics Year 2 > Optics > Flashcards

Optics Flashcards

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1
Q

What is Snell’s Law?

A

n1sinθ1 = n2sinθ2

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2
Q

What is the lens equation?

A

1 / u + 1 / v = 1 / f

Where:
* u is the object distance
* v is the image distance
* f is the focal length

(ur very fit)

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3
Q

What are optical resonators, also known as optical cavities?

A

• An arrangement of mirrors (or other optical elements) that allows a beam of light

• to circulate in a closed path

• at certain resonant frequencies.

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4
Q

What is the equation for the Q-factor of a gas laser cavity?

A

Q = f0 T(rt) (2π/L)

Where:
* f0 is the resonant frequency and
* f0 = c / λ

  • T(rt) is the cavity round-trip time and
  • T(rt) = 2L / c
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5
Q

For particle sizes equal to or larger than a wavelength, ______ scattering predominates.

A

For particle sizes equal to or larger than a wavelength, (Mie) scattering predominates.

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6
Q

Mie Scattering

______ interactions of ______ and ______ rays that result in ______ and ______ interference along different ______.

A

Mie Scattering

(Complex) interactions of (scattered) and (refracted) rays that result in (constructive) and (destructive) interference along different (paths).

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7
Q

nλ = ?

A

nλ = dsinθ

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8
Q

What constitutes a diffraction grating?

A

A large number of parallel, closely spaced slits.

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9
Q

What is the resolving power of a diffraction grating?

A

R = λ / Δλ

Where Δλ is the smallest resolvable wavelength difference.

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10
Q

______ is what happens with small holes and point light sources.

For example, _______.

A

(Interference) is what happens with small holes and point light sources.

For example, (Young’s Slits experiment)

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11
Q

______ is what happens with finite and small holes and finite light sources.

For example, ______.

A

(Diffraction) is what happens with finite and small holes and finite light sources.

For example, (Single-slit, Fraunhofer diffraction)

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12
Q

Give two examples of interference.

A

Young’s Slits experiment

Thin-film interference

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13
Q

Give two examples of diffraction.

A

Single-slit (Fraunhofer) diffraction

Near-field (Fresnel) diffraction

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14
Q

What is the equation for a travelling wave?

A

y(x, t) = A sin [kx - ωt + φ]

Where:
* A is the amplitude
* k is the wave number
* ω is the angular frequency
* φ is the phase

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15
Q

Superposition of Waves

If φ = 0, _______ interference takes place. The waves ______ and have amplitude _____.

A

Superposition of Waves

If φ = 0, (constructive) interference takes place. The waves (add) and have amplitude (2A).

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16
Q

Superposition of Waves

If φ = π, _______ interference takes place. The waves ______ and have amplitude _____.

A

Superposition of Waves

If φ = π, (destructive) interference takes place. The waves (cancel out) and have amplitude (zero).

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17
Q

How is the Q-factor of a resonant optical cavity related to the resonant frequency (f0)?

A

Q = (2πf0) x (Energy Stored) / (Power Loss)

In other words, Q is the ratio of the stored energy to the energy dissipated per radian of the oscillation.

Where f0 is the resonant frequency.

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18
Q

How is the Q-factor of resonant optical cavity related to the bandwidth (Δf)?

A

Q = f0 / Δf

Resonance bandwidth: the ratio of the resonance frequency f0 and the full width at half-maximum (FWHM) bandwidth Δf of the resonance.

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19
Q

Describe what is meant by Rayleigh scattering.

A

Rayleigh scattering is the scattering of molecules to very tiny particles.

It favours very small wavelengths.

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20
Q

Give an example of Rayleigh scattering.

A

The blue colour of the sky, since blue light is scattered slightly more efficiently than red.

You see light from Rayleigh scattering off the air.

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21
Q

Briefly describe the differences between the cases of Fresnel and Fraunhofer diffraction.

A

Distance between slit, and source.

Fresnel:
- Finite distance from slit
- Cylindrical or spherical wavefront considered

Fraunhofer:
- Infinite distance from slit
- Plane wavefront considered

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22
Q

What equation would you use to find the thickness of a film that is used to create constructive interference with a given value for the refractive index of the material and the wavelength of light?

A

2µt = (m + 1/2)λ

Where:
µ is the refractive index
t is the thickness
m is the order (1, 2, 3…)
λ is the wavelength

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23
Q

What equation would you use to find the thickness of a film that is used to create destructive interference with a given value for the refractive index of the material and the wavelength of light?

A

2µt = (m)λ

Where:
µ is the refractive index
t is the thickness
m is the order (1, 2, 3…)
λ is the wavelength

24
Q

Describe what is meant by spatially coherent light.

A

Spatial coherence tells us how uniform the phase of the wave front is.

It is a measure of the correlation of a light wave’s phase at different points transverse to the direction of propagation.

25
Q

What is Brewster’s angle?

A

tan(θb) = ne / n0

Where:
θb = Brewster’s angle (tilt angle to horizontal)
ne = refractive index of second medium
n0 = refractive index of first medium

26
Q

How would you find the thickness of a quarter-waveplate?

A

t(1/4) = (1/4) x (λ) / (ne - n0)

27
Q

How would you find the thickness of a half-waveplate?

A

t(1/2) = (1/2) x (λ) / (ne - n0)

28
Q

How does subtractive colour work?

A

Subtractive primaries relfect two of the RGB.
This means that only one of the RGB is absorbed.

29
Q

How does additive colour work?

A

Additive primaries relfect one of the RGB.
This means that two of the RGB are absorbed.

30
Q

Additive Colour System

Red + Green = ?

A

Additive Colour System

Red + Green = Yellow

31
Q

Additive Colour System

Red + Blue = ?

A

Additive Colour System

Red + Blue = Magenta

32
Q

Additive Colour System

Blue + Green = ?

A

Additive Colour System

Blue + Green = Cyan

33
Q

Give the equation for angular magnification for a telescope.

A

M = -fo / fe

Where:
fo is the focal length of the objective lens
fe is the focal length of the eyepiece

34
Q

What is the wavelength of red light?

A

λ(red) ~ 650nm

35
Q

What is the wavelength of green light?

A

λ(green) ~510nm

36
Q

What is the wavelength of blue light?

A

λ(blue) ~450nm

37
Q

How do you find the critical angle?

A

sin(θc) = ne / n0

38
Q

How would you find the coherence time for a given frequency bandwidth?

A

t(c) = 1/f

39
Q

How do you find the coherence length of a laser beam?

A

L = c t(c)

Where:
L is the coherence length
c is the speed of light
t(c) is the coherence time

40
Q

How would you find the position (distance from central fringe) of a maximum for Young’s Double Slit experiment?

A

y = mλD/d

Where:
m is the order
λ is the wavelength of light
D is the distance between the slits and the screen
d is the slit distance

41
Q

Describe briefly what is meant by Fresnel diffraction.

A

Fresnel diffraction is a “near-field” phenomenon,

which occurs when the usual approximations used for Fraunhofer diffractions, such as:

  • parallel rays from a point-source
  • the screen distance being > > aperture size and > > λ

no longer hold.

42
Q

Describe what is meant by temporal coherent light.

A

Temporal coherence is a measure of

the average correlation between
• the value of a wave and
• itself delayed by τ

at any pair of times

43
Q

How would you find the angle of a first order diffraction with an angle of incidence?

A

nλ = d(sinθ - sinθi)

44
Q

What does NA stand for?

A

Numerical Aperature

45
Q

What is the formula for NA?

A

NA = n sinθ

46
Q

What is the equation for microscopic resolution?

A

Δx = λ / 2(NA)

47
Q

What should you assume the wavelength of light is?

A

550nm

48
Q

How do you calculate birefringence?

A

ne - n0

49
Q

Why do most optical fibre telecoms systems operate in the infra-red part of the EM spectrum?

A
  • Rayleigh scattering is an important component of the scattering of optical signals in optical fibres
  • Silica fibres are disordered materials, thus their density varies on a microscopic scale
  • The density fluctuations give rise to energy loss
    due to the scattered light
50
Q

Describe a common practical application for the phenomenon of polarisation by reflection.

A

Polarized sunglasses use the Brewster’s angle to eliminate glare from the sun.

In a large range of angles around Brewster’s angle, the reflection of p-polarized light is lower than s-polarized light.

51
Q

Describe how the diameter (aperture) of a telescope is a critical parameter when considering the capabilities of the instrument.

A

The aperture of a telescope refers to the objective lens.

A larger objective lens creates a larger magnification.

Whilst high magnification makes an object appear larger,

light gathered by the scope is spread over a larger area

creating a fainter image

52
Q

What is Malus’ Law?

A

I(θ) = I(0) cos^2θ

Where:
I(0) is the initial intensity
I(θ) is the intensity due to the angle

53
Q

What is normal dispersion?

A

It is generally obsereved that the refractive index decreases with increasing wavelength.

This is called normal dispersion.

54
Q

What is anomalous dispersion?

A

In the small wavelength ranges, there is often increase of refractive index with increase of wavelength, due to absorption of radiation.

This is called anomalous dispersion.

55
Q

What is the Rayleigh criterion for a circular aperture?

A

sinθ = 1.22λ / D

Where:
θ is the angular resolution (radians)
λ is the wavelength of light
D is the diameter of aperture

56
Q

What is the Rayleigh criterion for a single slit?

A

sinθ = λ / D

Where:
θ is the angular resolution (radians)
λ is the wavelength of light
D is the diameter of aperture

Engineering Physics Year 2 (6 decks)

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