Waves

Question 1

What is the equation for wave speed?

Answer 1

Wave speed (ms^-1) = frequency (Hz) x wavelength (m)
c = f λ

Question 2

What is the equation for frequency?

Answer 2

Frequency (Hz) = 1 / time period (s)

f = 1/T

Question 3

What happens when a wave is reflected?

Answer 3

The wave hits the boundary at the angle of incidence, and bounces away at the angle of reflection

Question 4

What happens when a wave is refracted?

Answer 4

The wave hits the boundary and slows down, bending towards the normal

Question 5

What happens when a wave is diffracted?

Answer 5

The wave travels through a gap between two barriers and spreads out - the narrower the gap the more it spreads

Question 6

How many radians is equal to 360°?

Answer 6

2π radians

Question 7

What is phase?

Answer 7

The phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the start of the cycle

Question 8

What is phase difference?

Answer 8

The phase difference between two particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the two particles

Question 9

What is the equation for phase difference (in radians)?

Answer 9

2π x distance between points (m) / wavelength (m)

2πd/λ

Question 10

Define wavefront

Answer 10

The width of the wave, perpendicular to the direction of the wave

Question 11

What are coherent waves?

Answer 11

Waves are coherent if they have equal frequencies and a constant phase difference

Question 12

What is superposition?

Answer 12

Superposition is where two waves meet, interfere and create a stationary wave

Question 13

What are the types of interference?

Answer 13

Constructive interference - where the two waves combine to make supercrests and supertroughs, this happens when the waves are in phase

Destructive interference - where the two waves cancel each other out, this happens when the waves are out of phase

Question 14

What are stationary waves?

Answer 14

• Waves formed from the superposition of two progressive waves
• Nodes - points which don’t move, every half a wavelength between antinodes
• Antinodes - points which move the maximum displacement, every half a wavelength between nodes
• All points between two adjacent nodes are in phase to each other

Question 15

Frequency in stationary and progressive waves

Answer 15

Stationary waves
All particles except the nodes vibrate at the same frequency

Progressive waves
All particles vibrate at the same frequency

Question 16

Amplitude in stationary and progressive waves

Answer 16

Stationary waves
The amplitude varies from 0 at the nodes to a maximum at the antinodes

Progressive waves
The amplitude is the same for all particles

Question 17

Phase difference between two particles in stationary and progressive waves

Answer 17

Stationary waves
Equal to mπ, where m is the number of nodes between the two particles

Progressive waves
Equal to 2πd/λ

Question 18

What is the first harmonic?

Answer 18

The pattern of stationary waves on a string when it vibrates at its lowest possible frequency - nodes at each end of the string and an antinode in the middle

Question 19

Frequency of vibrations in a string

Answer 19

First harmonic frequency = 1/(2 x length of string) x sqrt(tension/mass per unit length)
f = 1/2L * sqrt(T/μ)

Question 20

What is a polarised wave?

Answer 20

A wave that oscillates in only one direction
Only transverse waves can be polarised
Polarised cameras and sunglasses use polarisation filters to reduce glare
TV and radio signals are polarised which means that the receiving aerial must be aligned in the same plane of polarisation to receive the signal at ​full strength

Question 21

Young’s double slit experiment

Answer 21

A coherent light source is shined through 2 slits so that it diffracts
The two new sources of light interfere:
Where they meet out of phase they destructively interfere, making a dark fringe
Where they meet in phase they contructively interfere, making a light fringe

fringe spacing = wavelength x distance between slits and screen / slit separation
w = λD/s

Question 22

Diffraction grating

Answer 22

A diffraction grating is a slide containing many ​equally spaced slits very close together

distance between slits x sin(angle between maximum and normal) = order of maximum x wavelength
dsinθ = nλ

Question 23

Refractive index

Answer 23

A ​refractive index​ is how much a material refracts light passing through it

refractive index = speed of light in vacuum / speed of light in substance
nₛ = c/cₛ

Question 24

Snell’s law

Answer 24

Refractive index of substance 1 x sin(angle of incidence) = refractive index of substance 2 x sin(angle of refraction)
n₁sinθ₁ = n₂sinθ₂

Question 25

Critical angle

Answer 25

The angle of incidence when the angle of refraction is 90 so the light passes along the boundary line; any angle higher than this will not cause refraction but instead total internal reflection

sin(critical angle) = refractive index of substance 2 / refractive index of substance 1
sinθ₁ = n₂/n₁

Question 26

Total internal reflection

Answer 26

Where the light reflects back at the boundary

Two conditions for this to happen:
The light must travel from a more optically dense to a less optically dense medium - from higher refractive index to lower refractive index
The angle of incidence of the light ray must exceed the critical angle

Question 27

Optical fibres

Answer 27

Optical fibres make use of total internal reflection to transfer signals

They consist of:
A core made of a high refractive index material
Cladding made of a low refractive index material

Light passes through the core and the difference in refractive indexes of the core and cladding allow for total internal reflection of the light

Question 28

Cladding in optical fibres

Answer 28

Provides tensile strength to the optical fibre - won’t break when twisted
Prevents information from transferring between different cores in a bundle
Prevents the core from being scratched - a scratch could lower the angle of incidence under the critical angle and if water gets in it could raise the refractive index above that of the core

Question 29

Modal and material dispersion

Answer 29

Modal dispersion
Waves go off in different directions and arrive at the end at different times, having gone different distances
Reduced by having a narrower core

Material dispersion
White light is dispersed due to the speed of light in glass depending on the wavelength
Using monochromatic light prevents this from happening