3. Waves

Question 1

What is a progressive wave?

Answer 1

The transfer of energy from one place to another without transferring matter.

Question 2

What is the amplitude?

Answer 2

The maximum displacement of the particles from the equilibrium position, measured in meters.

Question 3

What is the wavelength?

Answer 3

The length of one complete wave cycle, measured in meters.

Question 4

What is the time period?

Answer 4

The time taken for one complete wave cycle to happen, measured in seconds.

Question 5

What is the frequency?

Answer 5

How many complete waves occur in one second, measured in Hz.

Question 6

What is wave speed?

Answer 6

The speed of the wave (for EM waves this is c), measured in m/s.

Question 7

What is phase difference?

Answer 7

How synced points on waves are, measured in rads.

Question 8

How do you convert from radians to degrees?

Answer 8

2π rads = 360°

Question 9

What is path difference?

Answer 9

The difference in how far two waves have travelled, measured in λ.

Question 10

What is a longitudinal wave?

Answer 10

Waves which oscillate parallel to the direction of travel.
Compressions and rarefactions.

Question 11

What are transverse waves?

Answer 11

Waves which oscillate perpendicularly to the direction of travel.

Question 12

What does polarisation do?

Answer 12

It restricts the oscillations of a wave to one plane, only allowing light to oscillate in the same direction as it.

Question 13

What can polarisation prove?

Answer 13

If a wave is transverse or longitudinal.

Question 14

What are some applications of polarisation?

Answer 14

Reduces glare in glasses.
TV and radio signals​, which are usually plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at ​full strength.

Question 15

What is superposition?

Answer 15

The process by which two waves combine into a single wave form when they overlap.
This can be constructive or destructive interference.

Question 16

What is a standing wave? A graphical explanation is also required.

Answer 16

The formed by two waves travelling in opposite directions with the same frequency, wavelength, and amplitude.

Question 17

How can you experimentally find a standing wave?

Answer 17

Fix a tight piece of string at both ends then send a wave along it using a vibration generator. The signal will reflect at the other end and be sent back with a similar frequency.

Question 18

What are nodes?

Answer 18

Positions on standing waves which combine to give zero displacement.

Question 19

What are antinodes?

Answer 19

Positions on standing waves which combine to give maximum displacement

Question 20

What is the fundamental frequency?

Answer 20

The first harmonic, or standing wave, that can be found. It will show two nodes and an anti node. f = f0, λ = 2L.

Question 21

What is coherence?

Answer 21

Waves which are of the same frequency, wavelength, (polarisation, and amplitude) and in a constant phase relationship.

Question 22

What is constructive interference?

Answer 22

The path difference between the waves is a whole number of wavelengths, so they arrive in phase and add together giving a larger wave.

Question 23

What is destructive interference?

Answer 23

The path difference between the waves is a half number of wavelengths, so they arrive out of phase and cancel out leading to no wave at all.

Question 24

Young’s Double Slit experiment

Answer 24

It demonstrates the interference of light from two sources.
Shine a coherent light source through two slits about the same size as the wavelength (for maximum diffraction). This will create a pattern of light and dark fringes.

Question 25

What happens if you use white light instead of monochromatic light in the double slit experiment?

Answer 25

White light gives wider maxima (twice as wide as the other fringes) and a less intense diffraction pattern with a central white fringe and then alternating bright fringes which have violet closed to the centre and red furthest.

Question 26

What are precautions related to using lasers?

Answer 26

-wear safety glasses
-don’t look directly at the laser
+don’t shine it on reflective surfaces
-display a warning sign when it is on

Question 27

Diffraction diagrams

Question 28

What happens if you increase the number of slits in a diffraction grating?

Answer 28

The pattern is sharper and brighter.

Question 29

Derivation of dsinθ=nλ

Answer 29

1. Consider the first order maxima, where the path difference between two adjacent rays of light is one wavelength, denote the angle between the normal to the grating as θ.
2. You can then form a right angled triangle with side lengths of d and λ.
3. Use trigonometry.
4. Generalise the equation by substituting λ for nλ.

Question 30

What are some uses of line spectra?

Answer 30

You can identify which elements are present in stars. X-ray crystallography which can be used to measure the atomic spacing in certain materials.

Question 31

What is a refractive index?

Answer 31

A measure of how much a material slows down the light passing through it, found by dividing c by the speed of light in the substance.

Question 32

What does a high refractive index indicate?

Answer 32

A more optically dense material.

Question 33

What is the refractive index of air?

Answer 33

~1

Question 34

When does refraction occur?

Answer 34

When a wave enters a different medium, causing it to change direction.
If the material is more optically dense, the light ray bends towards the normal and vice versa.

Question 35

What is the critical angle?

Answer 35

The angle of incidence which has an angle of refraction of 90°, so the light is refracted along the boundary, where n1>n2.

Question 36

What is total internal reflection?

Answer 36

When the angle of incidence is greater than the critical angle (and n1>n2).

Question 37

What are some applications of total internal reflection?

Answer 37

Optical fibres: flexible, thin tubes which carry information in the form of light. They have an optically dense core surrounded by cladding of a lower optical density. The cladding also prevents the core from becoming damaged and prevents signal degradation through light escaping the core which can cause information to be lost.

Question 38

What causes signal degradation? (+diagrams)

Answer 38

-absorption: where part I’d the signal’s energy is absorbed by the fibre, reducing the amplitude of the signal.
-dispersion: this causes pulse broadening. Broadened signals can overlap causing a loss of information. There are two types:
-modal: caused by rays entering the fibre at different angles, so they take different paths leading to different arrival times. This can be reduced by making the core very narrow (reducing the possible differences in path lengths.)
-material: caused by light consisting of different wavelengths, meaning they will travel at different speeds along the fibre. This can be prevented by using monochromatic light.

Both absorption and dispersion can be reduced by using optical fibre repeaters, which regenerate the signal.

Question 39

Go through test