C12 Waves 2

Question 1

Interference if two waves are in phase or in antiphase

Answer 1

• If in phase, the waves will constructively interfere (increasing their intensity, e.g. sound/brightness)
• If in antiphase, destructive interference. If the waves have equal amplitude the wave cancels out completely.

Question 2

Coherence

Answer 2

When waves emitted from two sources have a constant phase difference, and the same frequency.

Question 3

What is maxima and minima

Answer 3

in an interference pattern, a maxima is a point where waves interfere constructively.
A minima is where they interfere destructively.

Question 4

Path difference

Answer 4

The difference in the distance travelled by two waves from their respective sources to a common point.

Question 5

What does the path difference of waves from coherent sources mean?

Answer 5

• If at a point two waves have a path difference of 0 or a multiple of nλ, they are in phase and constructively interfere.
• If the path differences is a multiple of (n + 1/2) λ they arrive in antiphase and destructively interfere.

Question 6

How to convert from path difference to phase difference

Answer 6

Double the path difference (as there are two waves to account for)

Question 7

Central maxima

Answer 7

Path difference is zero, the point is at equal distance from both sources, phase difference is 0.

Question 8

How to produce coherent microwaves

Answer 8

• Fire microwaves from a single source at a double slit.
• The waves will diffract around each slit, and each slit will act is its own microwave source, both being coherent.

Question 9

Young double-slit experiment

Answer 9

• Fire a monochromatic source of light (use colour filter) at a single slit.
• The light diffracts around the slit and then passes through double slit.
• As each of the double slit acts as a source of coherent waves, the interference pattern can be observed on a screen as alternating regions of darkness and brightness (fringes).

Question 10

What did Young’s double-slit experiment demonstrate?

Answer 10

The wave nature of light, as opposed to the previously thought theory by Isaac Newton that light was a stream of tiny particles.

Question 11

Stationary Wave

Answer 11

When two waves with the same frequency (and ideally amplitude) travelling in the opposite directions superpose.

Question 12

Nodes and antinodes

Answer 12

• Points of zero displacement (antiphase superposition) are nodes.
• Points of max displacement (in phase superposition) are antinode.

Question 13

Formula to calculate the wavelength of the light used to form an interference pattern in the double-slit experiment

Answer 13

λ = ax/D
a = distance between slits
x = fringe separation (distance between adjacent bright/dark fringes)
D = distance between double slit and screen
distance between slits must be much smaller than distance from slits to screen