Topic 4: Waves

Question 1

Define displacement (x).

Answer 1

The distance the medium has moved from the equilibrium position in a specific direction (m).

Question 2

Define amplitude (x₀).

Answer 2

The maximum displacement from the equilibrium position (m).

Question 3

Define frequency (f).

Answer 3

The number of oscillations per second (Hz).

Question 4

Define period (T).

Answer 4

The time for one oscillation (s).

Question 5

Define wavelength (λ).

Answer 5

The shortest distance between two points that are in phase along a wave (m). Eg. crest to crest.

Question 6

Define wavespeed (v).

Answer 6

The distance travelled per unit time by the energy of the wave (or by a wavefront) (ms⁻¹).

Question 7

Define phase (φ).

Answer 7

A measure of how “in step” different particles are.

Question 8

When does simple harmonic motion (SHM) occur?

Answer 8

SHM is a type of oscillation that takes place when the acceleration of (and the force on) an object is;
- proportional to its displacement from the equilibrium position.
- in the opposite direction to the displacement (i.e. towards the equilibrium position).

*The motion is due to a restoring force.

Question 9

If a graph of acceleration against displacement is plotted what would a straight line and negative gradient show?

Answer 9

• A straight line shows that acceleration and displacement are proportional.
• A negative gradient shows that acceleration and displacement are in opposite directions.

Question 10

Define wave.

Answer 10

A movement of energy through a medium.

Question 11

Describe the two types of wave motion.

Answer 11

Longitudinal waves - the particles of the medium vibrate parallel to the direction of the energy transfer. Eg. sound, earthquake P waves. Rarefaction and compression.

Transverse waves - the particles of the medium vibrate at right angles to the direction of the energy transfer. Eg. light, earthquake S waves. Crest and trough.

Question 12

Define intensity.

Answer 12

The power per unit area received by an observer. Measured in Wm⁻².

Question 13

What is the intensity of a wave related to?

Answer 13

It is proportional to the square of the amplitude of the wave. I∝A²
It is inversely proportional to the square of the distance from the source. I∝x⁻²

Question 14

Explain the two types of waves.

Answer 14

Mechanical waves. These require a medium to travel through. Eg. sound and earthquake waves.

Electromagnetic waves. These do not require a medium to travel through i.e. they can pass through a vacuum. All electromagnetic waves are transverse and travel at the speed of light in a vacuum.

Question 15

Give the order of magnitude for the electromagnetic spectrum.

Answer 15

(Roman Men Invented Very Useful Xray Guns)
In order of decreasing wavelength:
Radiowaves 10²
Microwaves 10⁻³
Infrared 10⁻⁵
Visible 10⁻⁷
Ultraviolet 10⁻⁹
Xray 10⁻¹²
Gamma rays 10⁻¹⁴

Question 16

Define the refractive index (n).

Answer 16

A measure of how fast a wave travels in the medium. The higher the value of n, the slower the wave travels.

Question 17

Give Snell’s Law.

Answer 17

n = sini/sinr
*Only applies if the first medium is air. If the first medium is not air, the law given in the data booklet applies.

Question 18

When does total internal reflection occur?

Answer 18

When light is travelling from a more to a less optically dense medium and the angle of incidence at the boundary is greater than the critical angle.

Question 19

Give the formula to work out the critical angle or refractive index.

Answer 19

sinθc = 1/n₁

Question 20

Define wavefront.

Answer 20

A line connecting points on a wave with the same phase/displacement.

Question 21

How is a ray drawn on a wavefront?

Answer 21

A ray is a line drawn to represent the direction a wave is travelling when viewed from above. It is drawn at 90° to the wavefronts.

Question 22

Define diffraction.

Answer 22

When waves move past an object or through a gap the waves tend to spread out. This is called diffraction.

Question 23

What is affected in refraction?

Answer 23

Speed and wavelength are affected.
Frequency stays the same.

Question 24

What is affected in diffraction?

Answer 24

Amplitude decreases as the energy is spread out over a longer wavefront.
Frequency, speed and wavelength stay the same.

Question 25

Describe constructive interference.

Answer 25

When waves meet in phase, they will form a resultant wave with an amplitude equal to the sum of the two individual waves.

Question 26

Describe destructive interference.

Answer 26

When waves meet out of phase, they will cancel each other out to give a wave of zero amplitude (assuming both waves are of equal amplitude).

Question 27

Define the principle of superposition.

Answer 27

If two or more waves meet, the resultant displacement at any point is found by adding the displacements produced by each individual wave.

Question 28

What are antinodal and nodal lines?

Answer 28

Antinodal lines are drawn when waves meet in phase (constructive interference). Path difference = nλ. Antinodes are the points at max displacement.

Nodal lines are drawn when waves meet out of phase (destructive interference). Path difference = (n + 1/2)λ Nodes are the points at 0 displacement.

Question 29

What are the units the formula s = λD/d?

Answer 29

Metres.
s = distance between the bright fringes
λ = wavelength of the light
D = distance from the double slits to the interference pattern on a screen
d = distance between the double slits

Question 30

How do standing waves form?

Answer 30

When two waves of the same type meet. They must be;
- of the same amplitude
- of the same frequency
- travelling in opposite directions

*Note: Energy is not transferred in standing waves.

Eg. when a wave reflects back on itself from a boundary. The inverted reflected wave interferes with the forward moving wave to create a standing wave.

Question 31

What is polarisation?

Answer 31

If a wave is polarised, then all the oscillation of the medium is only in one plane. When light is polarised, the electric field vector is oscillating in one plane only.

*Note: only transverse waves can be polarised.

Question 32

How can polarised light be produced from unpolarised light?

Answer 32

• Reflection of a surface (Brewster’s Law)
• Using polarising filters (Malus’ Law)

Question 33

Discuss Brewster’s Law.

Answer 33

When light reflects off a surface, it will be polarised in the plane of the surface. n = tanθ

Question 34

When is polarisation 100% when light reflects of a surface?

Answer 34

If the angle between the reflected and refracted ray is 90°.

Question 35

Discuss Malus’ Law.

Answer 35

Light is passed through a polarising filter and then an analyser.

Question 36

What happens when unpolarised light is passed through a polariser?

Answer 36

The light intensity is reduced by half.

Question 37

nm -> m

Answer 37

divide by 10^9

Question 38

Define an optically active substance.

Answer 38

One that rotates the plane of polarisation of light that passes through it.

Question 39

What is the graph for SHM?

Answer 39

Upside down parabola.

Question 40

What is the graph for intensity against an angle?

Answer 40

Parabola with a minimum at 90° and a maximum at 180°.

Question 41

How do sunglasses work?

Answer 41

Sunglasses work by blocking the horizontally polarised light in order to stop light glare from reflected rays off of water. So they have vertical slits only.

Question 42

Describe interference with polarised light.

Answer 42

Two waves must have the same plane of polarisation and phase in order to produce an interference pattern. If waves of two different polarised light meet, a wave of a different plane of polarisation is produced.

Question 43

What type of wave are standing waves in pipes?

Answer 43

Sound waves which are longitudinal so the particles are vibrating horizontally.

Question 44

Do waves travel faster in deep water or shallow water?

Answer 44

Deep water.