Topic 5 - Waves and the Particle Nature of Light

Question 1

What is a wave’s amplitude?

Answer 1

The amplitude of a wave is its maximum displacement from its equilibrium position

The magnitude of the maximum displacement reached by an oscillation in the wave

Question 2

What is a wave’s frequency?

Answer 2

The frequency of a wave is the number of complete waves that pass a point per second

Question 3

What is meant by the period of a wave?

Answer 3

The period of a wave is the length of time taken for one complete wave to pass a given point

The time taken for one complete oscillation at one point on the wave

Question 4

How are frequency and period linked?

Answer 4

Frequency = 1/period

They are reciprocals of each other

Question 5

What is wavelength?

Answer 5

The distance between the same point on two adjacent waves (peak-to-peak or trough-to-trough)

Question 6

State the equation used to calculate a wave’s speed.

Answer 6

v = f λ

Speed = frequency x wavelength

Question 7

Describe a longitudinal wave.

Answer 7

Particle oscillations are parallel to the direction of the wave’s motion / movement of the wave energy

Consists of rarefactions (low pressure regions), and compressions (high pressure regions)

Question 8

Describe a transverse wave.

Answer 8

Particle oscillations are perpendicular to the direction of the wave’s motion / movement of the wave energy

Consists of peaks (maximum positive displacement), and troughs (maximum negative displacement)

Question 9

What is a progressive wave?

Answer 9

A progressive wave is one that transfers energy from one point to another without any transfer of matter

Transferring energy via oscillations

Question 10

What is a standing wave?

Answer 10

A wave that stores energy rather than transferring it from one place to another

Consists of oscillations in a fixed space, with regions of significant oscillation and regions with zero oscillation, which remain in the same locations at all times

Question 11

What is path difference a measure of?

Answer 11

Path difference is a measure of how far ahead one wave is compared to another

The difference in distance that two waves must travel from their sources to a given point

Question 12

What is a wave’s phase?

Answer 12

A wave’s phase at a given point is a measure of how far through its cycle the wave is. It is usually measured in radians, where a complete cycle is 2π

Question 13

What is the phase difference between two waves at a given point?

Answer 13

The phase difference is the difference between the phases of the two waves - in other words, it is a measure of the difference between how far each wave is through its cycle

Question 14

What happens when two waves meet?

Answer 14

The two waves will interfere with each other

Question 15

What happens when two waves meet in phase?

Answer 15

They will interfere and undergo constructive interference

Question 16

What happens when two waves meet in anti-phase?

Answer 16

They will interfere and undergo destructive interference

Question 17

How is a standing wave formed on a string?

Answer 17

• a wave reflects from a closed end meaning two identical waves are travelling in opposite directions down the same string
• at points where the waves meet in phase, constructive interference occurs and an anti-node is formed
• at points where the waves meet in anti-phase, destructive interference occurs and a node is formed

Question 18

What is a node?

Answer 18

A node is a point of minimum displacement - there is no movement from the equilibrium position

Nodes are regions on a stationary wave where the amplitude of oscillation is zero

Question 19

What is an antinode?

Answer 19

An antinode is a point of maximum displacement

Antinodes are regions on a stationary wave where the amplitude of oscillation is at its maximum

Question 20

What two factors does the speed of a transverse wave on a string depend on?

(Give the equation)

Answer 20

• tension
• mass per unit length of the string

v = √(T/μ)

Question 21

State the equation used to calculate intensity.

Answer 21

I = P/A

I - intensity
P - power
A - area

Question 22

What is the refractive index of a material through which light travels a speed of ‘v’?

Answer 22

n = c/v

Where c is the speed of light in a vacuum

Question 23

State the equation linking the refractive indexes and angles at an interface between two mediums.

Answer 23

n1sinθ1 = n2sinθ2

Question 24

What is the critical angle?

Answer 24

The angle of incidence for which the angle of refraction is 90° and all the light passes along the boundary between the mediums. Beyond this angle all light will be reflected

The critical angle is the largest angle of incidence that a ray in a more optically dense medium can have and still emerge into less dense medium. Beyond this angle, the ray will be totally internally reflected

Question 25

State the equation used to calculate a critical angle.

Answer 25

sinC = 1/n

Question 26

What is total internal reflection?

Answer 26

Total internal reflection is where all the light is reflected back at the boundary between two mediums. It occurs when light is incident at an angle greater than the critical angle. Total internal reflection requires two conditions to be met: - the ray is attempting to emerge from the more dense medium - the angle between the ray and the normal to the interface is greater than the critical angle

Question 27

What is the focal point of a converging lens?

Answer 27

The single point where the parallel rays of light entering the lens converge (intersect on the opposite side of the lens) Rays of light from a distant object will arrive at the lens parallel to each other. A converging lens will bring these rays together at a point (focal point)

Question 28

What is the focal point of a diverging lens?

Answer 28

Parallel light rays diverge and can be traced backwards until they intersect at a point (focal point). Rays of light from a distant object will arrive at the lens parallel to each other. A diverging lens will spread these rays apart so they will not meet at a point.

Question 29

What is the focal length of a lens?

Answer 29

The distance between the centre of the lens and the focal point

Question 30

What is the equation used to calculate the power of a lens?

Answer 30

Power = 1/focal length

Question 31

How do you calculate the total power of a combination of thin lens?

Answer 31

P = p1 + p2 + p3 + …

Question 32

What is a real image?

Answer 32

A real image is one that can be projected onto a screen and is always inverted. Real images are the consequence of light meeting at a focal point. A real image: - can be projected onto a screen - is always on the other side of the lens from the real object

Question 33

What is a virtual image?

Answer 33

Virtual images are the consequence of rays of light appearing to meet at a point. They cannot be projected onto a screen. A virtual image: - cannot be projected onto a screen - is on the same side of the lens as the real object

Question 34

State the equation used to calculate the magnification of an image.

Answer 34

Magnification = (image height) / (object height) M = I/A

Question 35

What is plane polarisation?

Answer 35

Plane polarisation is when the oscillations of a wave are restricted to a single plane. Polarisation refers to the orientation of the plane of oscillation of a transverse wave. If the wave is (plane) polarised, all its oscillations occur in one singular plane