2. Waves and Optics

Question 1

Definition of Amplitude

Answer 1

The maximum displacement of a particle from equilibrium position.

Question 2

Definition of frequency

Answer 2

The number of complete waves per second

Question 3

Definition of wavelength

Answer 3

The smallest distance between two adjacent vibrating particles that are in phase

Question 4

Definition of wave speed

Answer 4

v = λf

Question 5

Definition of phase

Answer 5

Measurement of the position of a certain point along the wave cycle

Question 6

Definition of phase difference

Answer 6

The difference between the two positions of certain points along the wave cycle

Question 7

Longitudinal Waves

Answer 7

Waves where the vibrations are parallel to the direction of energy transfer
E.g: sound waves, strings

Question 8

Transverse Waves

Answer 8

Waves where the oscillations of the particles are perpendicular to the energy transfer
E.g: strings, EM waves

Question 9

Definition of polarised wave

Answer 9

A wave that can only oscillations in one plane, and can only work if the oscillations are perpendicular to the energy propagation.

Question 9

Definition of polarised wave

Answer 9

A wave that can only vibrate in one plane, and can only work if the oscillations are perpendicular to the energy propagation.

Question 10

Define stationary waves

Answer 10

The superposition of two progressive waves travelling in opposite directions and are coherent and have the same amplitude

Question 11

What is the principle of superposition?

Answer 11

The displacement of the wave is the resultant vector sum of the individual displacement of the two waves.

Question 12

What is constructive interference?

Answer 12

When the two waves have displacement in the same direction

Question 13

What is destructive interference

Answer 13

When the two waves have displacements in the opposite direction

Question 14

What are antinodes

Answer 14

When the two waves meet in phase and constructively interfere to create regions of maximum amplitude

Question 15

What are nodes

Answer 15

When the two waves meet completely out of phase and destructively interfere to create regions of zero displacement

Question 16

What is the formula for the first harmonic?

Answer 16

f = 1/2l*(T/μ)^0.5

Question 17

Describe the formation of stationary waves on a string

Answer 17

A wave travelling down the string that’s fixed on one end will be reflected back at the same wavelength, frequency and amplitude, forming a stationary wave.

Question 18

What is the first harmonic on a string.

Answer 18

The lowest frequency at which a stationary wave is formed forming two nodes and one antinode, which is half a wavelength.

Question 19

Describe the formation of stationary wave on a microwave.

Answer 19

Created by reflecting a microwave beam on a metal plate.

Question 20

Definition of coherence

Answer 20

When a two waves have the same wavelength and frequency and a constant phase difference

Question 21

Why are lasers used in interference experiments

Answer 21

Because lasers are monochromatic and have a small range of wavelengths and a fixed (small range of frequencies) to create clear interference patterns.

Question 22

What is the equation for single slit diffraction?

Answer 22

W = λD/2s

Question 23

Describe Single slit diffraction

Answer 23

The greatest diffraction occurs when the gap is roughly the same as the wavelength
There’s a bright central fringe that’s twice as wide as the other fringes
Bright fringes are caused by constructive interference when the waves meet in phase as the path difference is a whole number.
Dark fringes are caused by destructive interference when the waves meet π out of phase as the path difference is 1/2 nλ

Question 24

Describe Young’s double-slit experiment

Answer 24

The slits act as a coherent point source. The light fringes occur due to constructive interference as the waves are in phase due to the path difference being a whole number of wavelengths. The dark fringes occur due to destructive interference of the waves which are π out of phase due to the path difference being a whole number and a half wavelengths.

Question 25

What is the young’s double-slit equation?

Answer 25

w = λD/s

Question 26

Interference pattern with a white light source

Answer 26

White light is a mixture of multiple wavelengths, so each of the wavelengths would be diffracted by a different amount. The central maxima would be white, but there would be a spectrum for the other fringes, and the fringes would be wider.

Question 27

Diffraction grating equation

Answer 27

dsinθ = nλ

Question 28

Explain total internal reflection

Answer 28

When the angle of incidence is greater than the critical angle, where the angle of reflection is 90° and the incident refractive index is greater than the refractive index of the boundary material

Question 29

Define refractive index

Answer 29

Measures how much the light slows down when passing through the material

Question 30

Explain Optical fibres

Answer 30

The optically dense core is surrounded by cladding with a lower refractive index for TIR to occur. Cladding protects the core from damage and prevents signal degradation from light escaping the core.

Question 31

Explain absorption

Answer 31

Where part of the signal’s energy is absorbed by the fibre and reduces the amplitude of the signal

Question 32

Explain modal dispersion

Answer 32

The signal received is broader than the original signal due to the light entering at different angles so there’s a range of time travel for the signals causing pulse broadening

Question 33

Explain material dispersion

Answer 33

Caused by using light of different wavelengths so they travel at different speeds.