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1
Q

State the condition at which the critical angle for a material falls, and derive it?

A

Total internal reflection occurs when the transmits from a high refractive index toa lower one. The corresponding angle of incidence is known as the critical angle. (to derive the equation use Snell’s law with the angle of transmission being 90˚)

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2
Q

Digital light projectors are examples of devices that make use of DMDs, What does DMD stand for ?

A

Digital Micromirror Device

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3
Q

What are phasors?

A

Phasors are lines which represent the amplitude and phase of waves. Where the length is proportional to the Amplitude and the angle it makes to the horizontal axis represents the Phase.

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4
Q

What is pleochroism?

A

It is selective polarisation dependent absorption of some colours.

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5
Q

How did the modern filament bulb improve on the original carbon filaments?

A

Spiralised tungsten filaments instead of the Carbon filament.
1. The tungsten filament acts like a short distance but has a large resistance.
2. Reduced evaporation of Tungsten in comparison to Carbon and any evaporation is concentrated at the end of the bulb.
3. It is filled with an argon/Nitrogen mixture which cools the filament, reducing the hot spots and increasing lifetime.

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6
Q

How does a modern filament bulb, incorporating the modifications by Langmuir, work?

A

When a current is passed through the filament the electrons within the atoms are excited to higher energy levels as the absorb the energy. The time they are excited for is very small and so when they return to their original energy levels they emit light in the form of a photon.

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7
Q

What does the Planck Blackbody Law tell us, and what problem in classical physics does it solve?

A

It tells us that the spectral spread of radiation is determined by the temperature of a body. It solves the UV catastrophe.

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8
Q

What fundamental four observations describe the photoelectric effect?

A
  1. Must be above a certain frequency for emission.
  2. Kinetic energy of electrons is proportional to frequency, if above threshold.
  3. Emission instantaneous
  4. Above threshold, current is proportional to intensity.
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9
Q

What two ideas did Einstein apply and how did they each contribute to explaining the photoelectric effect?

A

Light consisting of light quanta, later named as photos which are proportional to the frequency of the EM wave. Materials having a workfunction

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10
Q

Describe the configurations of the Michaelson interferometer’s mirrors that will produce straight-line fringes and circular fringes.

A

Directly across from the light source there is a fixed mirror. Perpendicular to the source there is a moveable mirror which is across from the observer.

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11
Q

What is coherence length? and how does it relate to producing interference?

A

Coherence length is the the propagation distance in which a wave remains it’s phase.

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12
Q

What are the two methods of producing interefrence?

A

Division of wavefront and division of Amplitude.

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13
Q

Calcite is an example of a material that exhibits an unusual optical effect, what is the name of this effect? And what property of this material causes this effect?

A

Birefringence caused by the anisotropy of materials. These materials usually transmit light as two rays, even when one is incident.

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14
Q

What is unpolarised light?

A

Light where the direction of the electric field varies at random between successive measurements at one point.

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15
Q

List the components of a prism spectrometer

A
  1. Prism
  2. Slit
  3. Collimator
  4. Telescope
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16
Q

What would substituting a diffraction grating for the prism in a prism spectrometer do to the resulting spectrum?

A

The resulting dispersion would be linear for a diffraction grating as the separation of different wavelengths of light is the result of constructive and destructive interference. Whereas the prism separates light on a basis of differing indices of refraction.

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17
Q

What is the effect of pressure on the spectral lines produced in the sun? Why does this happen?

A

The higher the pressure the broader the spectral lines and the smaller the pressure the narrower the spectral lines.This is caused by an increase in collisons when there is a greater pressure and the number of excited electrons is incresed causing more emissions.

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18
Q

How is an absorption spectrum produced in a lab?

A

Pass white light through a sample. Some of the white light will have just the right energy to excite the atoms of the sample and light of that particular colour will be absorbed.

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19
Q

What is the fundamental difference between interference and diffraction?

A

Interference happens from adding waves from a discrete number source whereas diffraction happens from by waves from a continuous source.

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20
Q

Describe the production of an interference pattern, specifying the conditions for producing interference and giving an example of an experiment demonstrating this effect.

A

To produce interference there needs to be a constant phase relationship between the waves. The waves need to be monochromatic. Young’s slits experiments demonstrates this effect.

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21
Q

Using phasors or otherwise, explain how diffraction produces patterns of maxima and minima.

A

Sketch a diagram of the phasors.

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22
Q

How is the corona around the moon produced?

A

It is caused by the diffraction of light in a cloud, with water droplets or ice crystals.

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23
Q

With reference to the Bohr model, explain the fine spectral lines of Hydrogen’s emission spectra

A

The electrons occupy discrete energy levels. These energy levels are labelled by quantum numbers. When atoms are excited the electrons move up an energy level. As the electron leaves the excited stage and falls back down to it’s original energy level it emits light in the form of a photon. This is what an emission spectra line is.

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24
Q

How do the spectral lines of Hydrogen’s emission spectra relate to the absorption spectra of Hydrogen?

A

The spectral lines of the emission spectra are the inverse of the spectral lines of the absorption spectra. with the emission lines at the same frequency as the absorption lines.

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25
Q

Describe how you would produce an absorption spectra?

A

Pass white light through a sample. Some of the energy will have just the right wavelength to excite the atom and light of that wavelength will be absorbed.

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26
Q

Describe how you would produce an emission spectra?

A

Putting a little of the material in a flame and using the flame to excite the atomic electrons. The excited electrons move up an energy level for a short period of time and then when they fall they emit their energy in the form of a photon.

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27
Q

Describe how the Fourier series can be used to represent complex waves

A

The Fourier series breaks more complex waves down into a sum of sines and cosines.

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28
Q

Define phase speed for any sine wave.

A

It determined by the motion of a point of constant phase.

29
Q

For what does the acronym LASER stand?

A

Light Amplification by Stimulated Emission of Radiation.

30
Q

Give a detailed description of how a laser works.

A

Lasing takes place in a cavity which has parallel reflectors at either end and where one transmits a small fraction.

31
Q

Describe optical activity, including what it is, the underlying cause of the behaviour and an experiment that makes use of it.

A

Materials with Optical activity rotate the direction of polarisation as the light propagates through. It’s cause is that left and right circularly polarised light have different propagating speeds meaning that they have different refractive indices.

32
Q

How does a twisted nematic LCD work?

A

A liquid crystal display uses crossed polarisers to produce the dark state and an electrically induced change of polarisation to produce the bright state. The alignment of molecules in the LCD is induced by a surfactant to produce a highly optically active cell. A small voltage is sufficient to re-align the molecules.

33
Q

What is a photomultiplier?

A

A photomultiplier multiplies the electrical output of a photodetector. It amplifies the first initial photoelectron pulse by a succession of dynodes. Detection is a two stage process; Radiation produces light via phosphor, light is then detected by a photomultiplier then amplified.

34
Q

Assuming you wished to use a photomultiplier to detect X-ray photons, describe the components of the photomultiplier and how they would function to allow you do that.

A

It consists of a photocathode, dynodes and an anode. When an x-ray photon hits the photosensitive surface it emits an electron through the photoelectrFic effect. This electron is accelerated towards the first dynode by a high voltage. When the electrons hit the first dynode it releases several secondary electrons. This electron cascade continues until the electrons hit an anode where they are collected and create an electrical signal.

35
Q

What is retinal?

A

Retinal is a chromophore in rhodopsin which absorbs light. When it absorbs light is alters it’s configuration from what it’s called cis state to the trans state.

36
Q

Describe how the human eye functions for human rod cells.

A

The retina had rods and cones. The protein rhodopsin is found in rods. Rhodopsin contains the chromophore retinal absorbs light. The rhodopsin winds up and down the rod. Upon absorbing the light, retinal changes from it’s cis configuration to it’s trans configuration, releasing an electron. This starts a cascading process which leads to an optic nerve signal.

37
Q

Describe the appearance of the Sun’s light as viewed through a prism spectrometer and what information we can gain from the kind of spectrum.

A

The sun’s spectrum would look continuous.

38
Q

What particular mathematical property of these waves makes them useful for describing arbitrarily complex waves? Name the technique at work.

A

The Fourier technique can be used to describe complex waves by summing the

39
Q

Describe constructive and destructive interference.

A

Constructive interference occurs when two waves are in phase. Destructive interference occurs when two waves are exactly out of phase.

40
Q

Considering Planck’s law, explain why no object ever appears to glow green when heated.

A

Planck’s law tells us that the spectral spread of the radiation of a star is dependent on temperature. And so even if a star has a peak wavelength of light which is in the range of the green light wavelength. It will produce almost as much red light and so our eyes will perceive this as white. So to perceive a green star it would have to emit light in only the green range of wavelengths which isn’t possible.

41
Q

What are defining properties of elliptically polarised light?

A

There is no necessary relationship between amplitudes of the field and there is no necessary phase relationship.

42
Q

How does a sheet of polaroid produce linearly polarised light?

A

It produces linear polarisation of ight by transmitting the electric vector along the axis of the polaroid and absorbing the perpendicular vector.

43
Q

What special conditions define circular polarisation?

A

When the x and y amplitudes are both equal and there is a phase difference of pi/2. Right circular polarisation, in which the E rotates clockwise looking back down the direction of propagation. Left-hand circular polarisation, rotation is anti-clockwise and can flip the direction of the Ey.

44
Q

A display enhancer is a certain type of polariser placed over a screen. What type of polariser is used and how does it work?

A

A circular polariser is used, and so when external light falls on the polariser it becomes circularly polarised before it reaches the display. The reflected light changes the handedness and so the reflected light is blocked by the polariser. Light generated from the display passes through the polariser and hence is seen without background reflected light.

45
Q

What is the optical lever effect?

A

When a mirror is tilted about an axis perpendicular to the plane of reflection. The reflected beam twists through twice the angle the angle of the tilt of the mirror, which is the optical lever effect.

46
Q

What colour representation system does a digital light projector use to produce colours and how does this colour system work?

A

RGB

47
Q

Define HSV, briefly explaining how it represents colour.

A

Hue- gives the colour, Saturation gives the purity of the colour, and Value gives the luminosity of the colour.

48
Q

What is metamerism and how does it relate to colour systems generally?

A

Metamerism is the phenomena where the same colour can be produced by many different spectral compositions. It underlies the workings of three colour matching.

49
Q

How does the arc lamp work?

A

The arc lamp was the first electric light, it works by passing a current through two rods of carbon and when the two rods of carbon are seperated an arc of brilliant light appears.

50
Q

With reference to Planck’s body law, explain why the arc lamp and the filament bulb must operate at high temperatures and why their comparatively low temperatures causes inefficiency.

A

Planck’s blackbody law tells us that the spread of radiation is dependent on the temperature of the body. For the arc lamp and the filament bulb to emit light is the visible spectrum they need to be at a high temperature. The lower the temperature the less light being radiated.

51
Q

Considering Planck’s law, explain why no object ever appears to glow green when heated.

A

For an object to appear green it would need to emit radiation in only the range of ‘green’ wavelengths which is impossible. When an object is heated it also emits red wavelengths of light and so when perceived the combination appears to be white.

52
Q

Using the Bohr model of the atom, explain how electrically excited hydrogen gas produces a spectrum of discrete bright lines

A

Atoms consists of discrete energy levels which are occupied by electrons. As Hydrogen gas becomes electrically excited electrons move energy levels for a short period of time before falling back down to the original energy level and emitting a photon. This is what causes the bright lines in the Hydrogen’s emission spectra.

53
Q

What is a gas mantle and how does it work?

A

The gas mantle is made from cotton fabric impregnated with 99% thorium and 1% cerium compounds. When this was set alight the fabric burns away leaving oxides of the compounds fused into a hard mantle. The mantle transformed gas lighting, producing incandescent light to rival that of electricity.

54
Q

What were the two drawbacks of carbon light bulbs?

A

Carbon filament bulbs had a short lifetime and needed a high voltage to produce light.

55
Q

What is birefringence?

A

Birefringent materials transmit light as two rays when only one is incident.

56
Q

What property of a material causes birefringence. Explain how birefringence happens.

A

Birefringent materials are anisotropic. In birefringent materials light travels through the medium as two rays. One of these rays is propagated by an expanding spherical wave, where the electric vector is perpendicular to the optic axis. The other ray propagates as an ellipsoidal wave, where the electric vector is parallel to the principle plane. As both rays travel at different speeds they both have different refractive indices and so the phase of both is different.

57
Q

How do birefringent materials produce colours under a polarising microscope?

A
58
Q

Why do we not typically see interference from two ordinary, unrelated light sources?

A

For light sources to interfere with each other they need to be coherent and monochromatic.

59
Q

Describe Young’s slits experimental set up.

A
60
Q

Explain the set up of the Michaelson interferometer.

A

It consists of two mirrors, a beam splitter, a compensating plate, a source and a detector. The two mirrors are placed perpendicular to each other but optically parallel.

61
Q

Explain how the Michaelson interferometer produces an interference pattern, explaining the contribution of each element.

A

The Michelson interferometer consists of two mirrors, a beam splitter, a compensation plate, a source and a detector. The light source is split into two parts using the beam splitter. These parts then propagate to each mirror before reflecting off them and travelling back towards the beam splitter. At the beam splitter as they are both from the same light source they are coherent meaning that they can interfere with each other. This interference pattern produced can then be interpreted using the detector.

62
Q

What is diffraction?

A

Diffraction is the spreading out of light from it’s defined optical path.

63
Q

What are the two simplifications introduced by the concept of Fraunhofer Diffraction.

A

When the wave source is at infinity there are plane waves.
When the diffraction pattern is at infinity there is a small apeture.

64
Q

What is dispersion?

A

Dispersion is the splitting of white light into it’s consecutive colours.

65
Q

What defect does dispersion cause in lens systems?

A

Chromatic aberration.

66
Q

Give an example of when dispersion is useful.

A

The prism spectrometer.

67
Q

What is the condition required for internal reflection?

A

Total internal reflection occurs when the light is being transmitted from a high refractive index to a lower one.

68
Q

What is an evanescent wave, and how does it arise?

A

When total internal reflection is taking place, a wavefield extends into the low refractive index medium. The wavefront doesn’t propagate perpendicularly to the surface but decreases exponentially.

69
Q

What is frustrated total internal reflection?

A

If a second surface is brought close to the total internally reflecting surface, into the evanescent wavefield. Then frustrated total internal reflection occurs where some energy crosses the narrow gap and propagation resumes in the neighbouring medium.

Light science (5 decks)

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