Waves

Question 1

What is a wave?

Answer 1

Something that transfers energy from one point to another without the transfer of material.

Question 2

What happens when a transverse wave enters a denser medium?

Answer 2

The speed and wavelength decrease but the frequency stays the same.

Question 3

What are longitudinal waves?

Answer 3

Waves where the direction of vibration of the particles is parallel to the direction the wave is travelling in. They contain compressions and rarefactions (expansions).

Question 4

What are transverse waves?

Answer 4

Waves where the direction of vibration is perpendicular to the direction in which the wave travels.

Question 5

What does “in phase” mean?

Answer 5

Where the peaks and troughs of two waves are aligned. Their phase difference is 360 or 2pi.

Question 6

What does “antiphase” mean?

Answer 6

Where the peaks of one wave line up with the troughs of another. The phase difference is 180 or pi.

Question 7

How to calculate phase difference in radians

Answer 7

2 X pi X distance between two points along a wave / wavelength

Question 8

What is an unpolarised wave?

Answer 8

A wave that vibrates in all planes

Question 9

What is a polarised wave?

Answer 9

A wave that vibrates in only one plane.

Question 10

Why can’t sound waves be polarised?

Answer 10

Because there is no effect in rotating the wave (it is the same all the way around). It oscillates parallel to the direction of its energy transfer (motion) as it is longitudinal.

Question 11

Uses of polarisation

Answer 11

Polaroid sunglasses reduce glare from reflected light.
Radio waves from a transmitter are polarised so the aerial must be aligned in the same plane as the waves to get the best reception.
3D glasses feed different images into either eye as the glasses are polaroid filters.

Question 12

How does a polaroid filter work?

Answer 12

When unpolarised light passes through the filter, the transmitted light is polarised as the filter only allows light through which vibrates in a certain direction according to the alignment of its molecules. If light enters two of the same filter but one is at 90 degrees to the other, no light can pass through.

Question 13

What happens when two waves meet?

Answer 13

They pass through each other. The total displacement of each point is the sum of the displacements at that point on each wave e.g where a trough meets a peak, the total displacement is 0.

Question 14

What is the period of a wave?

Answer 14

The time taken for one complete wave to pass a fixed point.

the reciprocal of frequency

Question 15

How do standing waves form?

Answer 15

Progressive waves are reflected from the two ends of the string.
They travel in opposite directions.
They have the same frequency and amplitude.
They superpose (combine) to form a standing wave.
ROFAS
At some points the waves always destructively interfere and forms nodes. Opposite for antinodes.

Question 16

Nodes and antinodes

Answer 16

Node: fixed point on a stationary wave pattern where the amplitude is 0(no displacement)
Antinode: fixed point on a stationary wave pattern where the amplitude is a maximum (between the nodes)

Question 17

What happens to the frequency and wavelength of stationary waves and you go up the harmonics?

Answer 17

Times wavelength of 1st harmonic by 1/number of harmonic.

Add the frequency of the 1st harmonic each time.

Question 18

When is the phase difference of two points on a stationary wave pi?

Answer 18

When there is an odd number of nodes between them.

Question 19

When is the phase difference of two points on a stationary wave 2pi?

Answer 19

When there is an even number of nodes between them.

Question 20

Difference between progressive and stationary waves

Answer 20

Progressive waves transfer energy from one place to another whereas stationary waves don’t, instead they store it in one place.

Question 21

Where is the highest energy in a wave?

Answer 21

At the antinodes.

Question 22

Define coherent

Answer 22

Waves that have the same frequency and a constant phase difference.

Question 23

Define path difference

Answer 23

Difference in distances from the two coherent sources to an interference fringe.

Question 24

Define superposition

Answer 24

Effect of two waves adding together when they meet to give a single wave.

Question 25

Constructive interference

Answer 25

Waves superposing to give a larger amplitude wave

Question 26

Destructive interference

Answer 26

Waves superposing to give a smaller amplitude wave or no wave at all.

Question 27

Describe the appearance of the interference pattern and explain how it is formed

Answer 27

There are equally spaced bright and dark fringes. The light is coherent from the two slits. Light diffracts through the two slits, the waves overlap and superpose. Where the light waves meet in phase, there is constructive interference and a bright fringe is seen. Where they meet in antiphase, there is destructive interference and a dark fringe is seen.

Question 28

How does the single slit make the light that passes through it coherent?

Answer 28

The single slit is a single source diffracting the light into the next two slits.

Question 29

Fringe width equation

Answer 29

Fringe width = wavelength/slit width X 2 X distance between screen and slit

w=y/a X 2D

Question 30

Define diffraction

Answer 30

The spreading out of waves when they pass through a gap or by an edge.

Question 31

What happens if light enters glass at the critical angle?

Answer 31

The angle of refraction is 90 degrees

Question 32

How do optical fibres work?

Answer 32

Light pulses enter above the critical angle and is therefore totally internally reflected and continues to be so as every time is hits the boundary, the angle of incidence exceeds the critical angle.

Question 33

Function of cladding in step index fibres

Answer 33

Protects the core from damage

Prevents leakage of light between fibres

Question 34

How do endoscopes work?

Answer 34

One bundle of fibres has light sent through it to illuminate an area inside the body. The light reflects back through the lens of the other bundle which is used to form an image on the other end of the bundle. This bundle needs to be a coherent bundle meaning that the fibre ends at each end are in the same relative positions.

Question 35

Material dispersion

Answer 35

Where white light pulse become longer by the end of an optical fibre because the different wavelengths travel at different speeds in glass (violet slowest). This means the violet component falls behind.

Question 36

Modal dispersion

Answer 36

Where, in a wide core, the rays arrive at the end at different times because they have taken different paths which have different lengths. The pulse would therefore become longer.

Question 37

Pulse broadening

Answer 37

The spreading of light pulses as they travel down a fibre.

Question 38

What is the displacement of a wave?

Answer 38

The distance and direction a vibrating particle is from its equilibrium position.

Question 39

Define amplitude

Answer 39

The maximum displacement of a vibrating particle

Question 40

Define wavelength

Answer 40

The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time.

Question 41

What is a cycle of a wave?

Answer 41

From maximum displacement to the next maximum displacement.

Question 42

Define frequency

Answer 42

The number of cycles of vibration of a particle per second. Or the number of complete waves passing a point per second.

Question 43

What causes waves to diffract more?

Answer 43

Narrower gap, longer wavelength

Question 44

Define interference

Answer 44

The formation of points of cancellation and reinforcement where two coherent waves pass through each other.

Question 45

Explain refraction

Answer 45

A wavefront meets a denser medium at an angle. One end of the wavefront meets it first and slows down. In the time taken for the other end to meet the denser medium, it has moved further as it was travelling faster than the other end. So the wavefront bends towards the normal.

Question 46

Why might the minimum intensity in a fringe pattern not be 0?

Answer 46

The amplitude of the two waves that superpose may not be equal

Question 47

Criteria for TIR

Answer 47

The incident substance has a larger refractive index than the other substance. The angle of incidence exceeds the critical angle.

Question 48

How does a diffraction grating work?

Answer 48

Monochromatiq light directed normally at it. Light is transmitted in certain directions only as the the light passing through each slit is diffracted and the diffracted light waves from adjacent slits reinforce each other in certain directions only and cancel out in all other directions. Zero order is central beam in same direction as incident beam.

Question 49

Derive the diffraction grating equation

Answer 49

2 beams perpendicular to grating and leave parallel to each other at an angle. Draw line from one slit to the other’s beam so the lines are perpendicular. Distance from the other slit and this point is nλ. Distance between slits is d. θ is angle from constructed line to diffraction grating. Maximum number of orders is value of d/λ to nearest whole number.

Question 50

How to investigate how frequency of first harmonic varies with tension in string

Answer 50

Hav one end of string connected to vibrator and other end connected to mass holder over a pulley. Vibrator is connected to signal generator. The weight of masses will be tension in string. Adjust frequency of vibrator until first harmonic stationary wave is formed. The relationship should be frequency directly proportional to root tension.

Question 51

How to investigate how fringe width varies with distance between slits and screen in Young’s slit experiment

Answer 51

Use laser of monochromatic light. Point laser towards double slit and measure the distance across 6 fringes produced on the screen behind. This is 5 fringe widths. Vary the distance between the slits and the screen. The fringe width should be directly proportional to this distance, D.