What two conditions must be satisfied for sources to be described as coherent?

They must have a constant phase difference and the same frequency.

What would happen to the interference pattern if the two sources in a superposition experiment had different frequencies?

No stable interference pattern would form because the sources would not be coherent (they would not maintain a constant phase difference).

Why does a single light source need to pass through a double slit rather than using two separate light bulbs to demonstrate interference?

Two separate light bulbs would not be coherent; using a single source ensures both slits emit light with a constant phase difference and the same frequency.

Define path difference.

Path difference is the difference in distance travelled by two waves from their sources to a given point.

What is phase difference and in what units can it be measured?

Phase difference is the fraction of a cycle by which one wave leads or lags another, measured in radians or degrees.

State the formula that relates phase difference to path difference.

Δφ = 2πΔx / λ, where Δφ is phase difference, Δx is path difference, and λ is wavelength.

OCR A-Level Physics A H556

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Waves

Interference, coherence and path difference

2 things to know
Papers 2 and 3

Overview

Recall questions on coherence, path difference and phase difference, the conditions for constructive and destructive interference, and why a single light source is used with a double slit.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Define phase difference and path difference, and state the formula that relates them.

Numbers

What phase difference in radians corresponds to a path difference of one wavelength?

Questions from this subtopic

Each one with the model answer underneath.

1
What two conditions must be satisfied for sources to be described as coherent?
Model answer
They must have a constant phase difference and the same frequency.
2
What would happen to the interference pattern if the two sources in a superposition experiment had different frequencies?
Model answer
No stable interference pattern would form because the sources would not be coherent (they would not maintain a constant phase difference).
3
Why does a single light source need to pass through a double slit rather than using two separate light bulbs to demonstrate interference?
Model answer
Two separate light bulbs would not be coherent; using a single source ensures both slits emit light with a constant phase difference and the same frequency.
4
Define path difference.
Model answer
Path difference is the difference in distance travelled by two waves from their sources to a given point.
5
What is phase difference and in what units can it be measured?
Model answer
Phase difference is the fraction of a cycle by which one wave leads or lags another, measured in radians or degrees.
6
State the formula that relates phase difference to path difference.
Model answer
Δφ = 2πΔx / λ, where Δφ is phase difference, Δx is path difference, and λ is wavelength.

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