A Level AQA Physics Waves and Optics

What is pulse broadening?

When the duration of a pulse increases because of dispersion. Each pulse lasts a longer time when it is received than when it was sent

Uses of polaroid material

To reduce unwanted reflections in photography and in polaroid sunglasses to reduce glare

The consequence of pulse broadening?

Overlapping pulses cannot be easily distinguished, limiting the maximum frequency and so limiting the data that can be sent

What is absorption?

When the energy of a signal is absorbed by the optical fibre in which it travels

The consequence of absorption

The further the light travels, the weaker the signal becomes

What is material dispersion?

The spreading of a signal caused by the variation of refractive index with wavelength

What is modal dispersion?

The spreading of a signal caused by rays taking slightly different paths in the fibre

What is a fibre optic?

A very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using TIR

What is the function of cladding?

Has a lower refractive index to keep most light in the core due to TIR at the cladding-core boundary. Prevents cross-talk if fibres touch and protects the core from scratches.

What is total internal reflection?

When the angle of incidence is greater than the critical angle refraction can't happen therefore, all light is reflected back into the material

Recall Snell's law:

n₁sinθ₁=n₂sinθ₂

What do the symbols in Snell's law stand for?

n₁ = refractive index of material 1
θ₁ = angle of incidence of ray in material 1
n₂ = refractive index of material 2
θ₂ = angle of refraction of ray in material 2

What is the refractive index of air?

approximately 1

What do the symbols in n=c/cₛ stand for?

n = refractive index of the material
c = 3 x 10⁸ ms⁻¹
cₛ = speed of light in material in ms⁻¹

Define the principle of superposition

When 2 or more waves pass through each other, the displacements due to each wave combine

Explain the function of polaroid material

If you view partially-reflected light through a polariser at the right angle, you can block out some of the reflected light. This reduces the intensity of light in your eye

Explain the alignment of aerials for transmission and reception

TV and radio signals are polarised by the orientation of the rods on the transmitting aerial. To receive a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting aerial. The rods on TV aerials are all horizontal.

Explain polarisation as evidence for the nature of transverse waves

Polarisation can only happen for transverse waves as the direction of propagation and the direction of vibration must be different

All electromagnetic waves __________

travel at the speed of light in a vacuum

Describe the direction of displacement of particles/fields relative to the direction of propagation in transverse waves

at right angles to the direction of propagation

Describe the direction of displacement of particles/fields relative to the direction of propagation in longitudinal waves

parallel to the direction of propagation

Explain the nature of electromagnetic transverse waves

They travel as vibrating electric and magnetic fields with vibrations perpendicular to the direction of energy transfer

How to represent transverse waves on a graph?

Graphs of displacement against distance along the path of a wave or they can be shown as graphs of displacement against time for a point as the wave passes, they often give the same shape so check the x axis

How to represent longitudinal waves on a graph?

Usually see them plotted as displacement against time these can be confusing though as they look like a transverse wave

Explain the nature of longitudinal waves

Consists of alternate compressions and rarefactions of the medium its travelling through

Examples of transverse waves

electromagnetic waves, waves on a string

Example of longitudinal wave

sound

How is phase difference measured?

angles (radians and degrees) or as fractions of a cycle

What is the phase difference in degrees and radians of distance λ?

360° 2π

Light bends towards the normal when __________

it enters a more optically dense medium e.g. air to glass

Why are optical fibres narrow?

to reduce modal dispersion

How do we reduce material dispersion?

by using monochromatic light

What does partially polarised mean?

some of the light vibrates along only one direction

Define total destructive interference

when 2 waves with equal and opposite displacements meet they cancel each other out completely

Define constructive interference

when two waves that are in phase overlap and form a bigger wave

Define phase difference

the amount by which one point on a wave lags behind another point on the wave

Define the refractive index

Ratio of the speed of light in a vacuum to its speed in a given material

the relationship between refractive index and optical density

the higher a mediums refractive index the higher its optical density

What is the critical angle?

The angle of incidence when the angle of refraction is 90 degrees

Define diffraction

the spreading of waves as they pass through a narrow gap or pass an obstacle

When is diffraction greatest?

when the wavelength is about the same size as the gap or obstacle width

Relationship between diffraction and wavelength

diffraction is proportional to wavelength

Relationship between refraction and wavelength

refraction is inversely proportional to wavelength

In single slit diffraction what causes the interference?

the path difference between the top of the slit and the bottom of the slit

Describe a diffraction pattern for single slit interference

central maximum is by far the brightest and twice as wide as the outer fringes, all outer fringes are the same width, fringes decrease in brightness the further from the centre

What are double slit peaks affected by?

the diffraction envelope, which is determined by the width of the single slits making up the grating

What does the 'W' in the single slit diffraction formula represent?

width of central maximum

What does the 'D' in the single slit diffraction formula represent?

distance between screen and slits

What does the 'a' in the single slit diffraction formula represent?

width of slit

Define diffraction grating

material with thousands of slits close together

Why is an interference pattern produced when using a diffraction grating?

a result of interference between many slits

What does a diffraction grating pattern look like?

a central fringe of maximum intensity, further maxima are produced when the path difference for light leaving 2 adjacent slits is a λ, 2λ etc.

What type of interference is present at bright fringes?

constructive

At the first order maximum what is the the path difference between adjacent slits?

λ

What does the 'n' in the diffraction grating formula represent?

order of maximum

What does the 'd' in the diffraction grating formula represent?

line spacing

What does the 'θ' in the diffraction grating formula represent?

angle between normal

How to find the highest order of maximum?

light cannot diffract by >90°

What is slit separation measured in?

m

How to find grating spacing?

1 x 10⁻³ m / number of lines per mm

What does more lines per mm mean?

the smaller the spacing, the fewer maxima visible, increases the resolution of the diffraction grating

Describe the diffraction grating pattern for white light

the central maximum is white, the 1st maximum produces the colours of the rainbow with blue closest to the central maximum and red furthest out, the fringes of each colour are narrower than the central maximum, 2nd and 3rd order maximum are a sequence of colours; fringe spacing increases with wavelength so colours being to overlap

What is diffraction used for?

measure wavelength of light, chemical analysis, astrophysics, to find the structure of crystalline materials

How are diffraction gratings used in fibre optics?

monochromators use different diffraction gratings to produce monochromatic light to avoid material dispersion

Define stationary (standing) wave

the superposition of 2 progressive waves with the same frequency (or wavelength) and amplitude, moving in opposite directions

How are stationary waves and progressive waves different?

no energy is transmitted by a stationary wave

Define nodes

points on the wave where the amplitude of the vibration is 0

Describe the stationary wave formed at the start of a cycle and 1/2 cycle for 2 waves moving opposite directions on a graph of displacement against position

a flat horizontal line in the middle, showing waves interfere destructively

Describe the stationary wave formed at 1/4 of a cycle and 3/4 cycle for 2 waves moving opposite directions on a graph of displacement against position

a wave of double the amplitude, showing waves interfere constructively

Define antinodes

points of maximum amplitude

What interference occurs at a node?

destructive, the displacements of the 2 waves always cancel each other out

What interference occurs at an antinode?

constructive, the displacements of the 2 waves combine to make a bigger displacement

Define first harmonic

lowest possible resonant frequency

How many loops does the first harmonic have?

1

How many nodes does the first harmonic have?

2

How many wavelengths does the first harmonic have?

1/2

How do the number of loops differ with each next harmonic?

1 extra loop for each harmonic

How do the number of nodes differ with each next harmonic?

1 extra node for each harmonic

How do the number of wavelengths differ with each next harmonic?

Add 1/2 wavelength for each harmonic

How does frequency differ between each harmonic?

increases by the value of the frequency of the 1st harmonic

Describe the waves inside a wind instrument

closed end forms a node, from which sound waves reflect. An open end forms an anti-node, where the amplitude is greatest

How to find the lowest frequency that can be played on a wind instrument modelled as a tube with one closed end and one open end given its length ?

f = c / λ where, c = 330 λ = length x 4

What speed do electromagnetic waves travel at in a vaccum?

3 x 10⁸ m/s

Define resonant frequency

When a complete number of 1/2 wavelengths fit in the length of the vibrating string.