# Waves and Optics

Waves questions based on the new AQA specification (2016 onwards). I hope it's useful!

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• Created by: LouiseG
• Created on: 28-04-16 20:42
What are longitudinal waves?
Waves in which the oscillations are parallel to wave propagation
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Give an example of a transverse wave.
Any EM wave; water waves; S waves
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What is polarisation?
When a wave is made to oscillate in one plane only (Only transverse - by preventing the oscillation of longitudinal waves, the whole wave energy would be absorbed as they are not "3D"/don't vibrate across planes)
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What is the amplitude of a wave?
The maximum displacement from equilibrium
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How is the period calculated?
1/frequency
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What is the wavespeed equation?
wave speed = wavelength x frequency
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What is 360o in radians?
2 pi
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What is 1/4 wavelength phase difference in radians?
1/2 pi (2 pi x 1/4 = pi/2)
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When are two points in antiphase?
When their phase difference is pi / 180o / they have a path difference of 1/2 wavelength
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What is diffraction?
The spreading out of waves past an obstacle or through a gap
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What is refraction?
The change of speed of a wave as it enters a new medium (and direction, if the incident wave hits the boundary at an angle other than 90o)
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What quantity DOESN'T change in refraction?
Wave frequency (wavelength and speed both change)
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What is the principle of superposition?
When two waves meet, the total displacement is equal to the sum of the individual displacements at that point.
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When a crest meets a crest...
... there is constructive interference, also called reinforcement. This creates a "supercrest".
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What is another word for cancellation?
destructive interference / negative reinforcement
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For waves to be coherent, what must be constant?
Their phase difference (as well as speed, frequency and wavelength; but not necessarily amplitude)
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Why, in a room with two stereo speakers, might there be areas of louder and quieter music?
The speakers produce coherent waves, with interfere with each other to produce standing waves - the quieter regions have destructive interference (nodes) and louder areas constructive.
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How is a standing wave formed?
From the interference of coherent waves travelling in opposite directions. The time taken for a wave to reflect from a boundary is equal to a whole number of oscillations of the vibration source
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How many nodes in the fundamental frequency for a standing wave on a string?
2 (one at each fixed end)
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Standing waves store _(a)_ and are formed of constant patterns of _(b)_ and _(c)_. The frequency of the first harmonic, where L is the string length, is _(d)_
(a) Energy ((b) Nodes (c) Antinodes (d) v/2L (The wavelength is twice the string's length, hence 2L. V is the wavespeed).
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A standing wave's appearance goes from maximum displacement at the antinodes to a string of zero displacement in all regions. What fraction of a cycle is this?
1/4
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What is the phase difference between two points separated by a node?
180o or pi (antiphase)
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Almost all particles in a standing waves vibrate at the same (a) , but at different (b).
(a) Frequency ('almost all' because the node particles do not vibrate at all) (b) Amplitude
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What is the frequency of the third harmonic in terms of v and L (length of the string?)
3v/ 2L (This is as one wavelength is two thirds of the string length (two of the loops) therefore v/(2/3L) is the same as 3v/2L )
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What is the frequency of the third harmonic in terms of F (Frequency of the first)?
3F
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What is Snell's Law?
n1 sin01 = n2 sin02 (0= theta, the angle in that medium; n = the refractive indexes)
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Why, if I have a ray travelling from water to glass to diamond, do I not need to work out the angles of incidence and refraction in the glass?
Because if n1 sin01 = n2 sin02 ; and n2 sin 02 = n3 sin03 ; then n1 sin 01 = n3 sin03 so the middle ratio can be cut out.
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What is the equation for the critical angle?
Sin 0c = n2 / n1 (Rearrangement of Snell's law, where Sin02 is just sin90; or 1 )
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What other quantity, other than angle, is used to calculate refractive indexes?
The speed of light in the media (specifically the ratio of the speed of light in a vaccum to the speed of light in the medium
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What are the two conditions for total internal reflection?
The incident substance has a larger refractive index than the other one (e.g., diamond-air but not air-diamond boundary) ; The angle of incidence exceeds the critical angle
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Give two uses for optical fibres
Endoscopes and in communications
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What is modal dispersion?
When light can take a variety of paths down a fibre's core so the signal becomes distorted as different waves arrive at different times.
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What is the solution to this?
Use a monomode (very, very thin!) fibre
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What is material dispersion?
Where the speed of the waves differs as different wavelengths are used (they have different critical angles) so signals broaden as violet light travels faster.
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What is the solution to this?
Use a source of monochromatic light (laser)
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What is the function of the cladding in optical fibres?
To prevent scratches (so light doesn't escape); to prevent "cross-talk" and protect data; to ensure TIR always happens (it's less optically dense than the core)
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What are the conditions for a bright fringe to form in Young's Double Slit experiment?
Light from both slits arrives in phase (i.e. a path difference of a whole number of wavelengths)
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What causes the dark fringes?
Cancellation - waves arrives out of phase (180o) so the light waves destructively interfere so no light is seen.
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What is the equation of fringe separation?
width of fridge = wavelength x distance to screen / slit spacing
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Red light is used instead of blue light. The fringes...
... are more widely spaced
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Why did Young need to use a single slit in front of the double slit?
He used white light, which is not coherent; so by diffracting it first through a single slit ensured the waves originated from the same point and were in phase upon hitting the double slits.
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Why should you never look directly at a laser?
The beam of coherent light is very concentrated in a small area. The eye focuses the beam further, on to the retina, which would be intense enough to destroy it.
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What is observed if white light is used?
The central fringe is white (all wavelengths have 0 path difference at the centre). A spectrum is formed for the fringes, with blue inner tinges and red outer. The dark fringes are imperceptible; but the colours become less distinct further out.
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What is the intensity variation for single slit?
The central maxima is twice as wide as the subsidiary maxima, which are sucessively less and less bright. The outer fringes are much less intense than the central fringe.
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What is a diffraction grating?
A plate with many closely spaced parallel lines running down it.
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How do you convert 600 lines/mm into the slit spacing, d ?
(1x10^-3) / 600
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What is the diffraction grating equation?
n x lambda = d sin0 (n= the order of light ; lambda = wavelength d = slit spacing sin0 = sine of the angle of the order)
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What is the equation to find the maximum order?
n = d / lambda (This is as the maximum order is smaller than sin90, because 90 is the furthest angle from the screen. sin 90= 1. Remember to round DOWN to find n).
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How are fractions of a degree expressed?
In "minutes", where 60' (60 minutes) = 1o (1 degree). So 14o 55' is 14.92 degrees
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Prove the diffraction grating equation.
(Difficult without a diagram but anyway...) Use a triangle where the hypotenuse is d, the slit spacing, the opposite side is n number of wavelengths (path difference between the two waves) and the angle theta is the order angle. Use sin0 = O/H
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What causes emission spectra?
When a heated gas emits specific frequencies of light, due to photons being emitted as electrons deexcite between characteristic energy levels.
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## Other cards in this set

### Card 2

#### Front

Give an example of a transverse wave.

#### Back

Any EM wave; water waves; S waves

### Card 3

#### Front

What is polarisation?

### Card 4

#### Front

What is the amplitude of a wave?

### Card 5

#### Front

How is the period calculated?