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C.3.6—Coherence

Syllabus
First assessment 2025
Objective
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Level
SL

Explain Coherent Sources

Define coherence

Two waves are coherent if they have the same frequency and a constant phase difference. The phase relationship does not drift with time.

Connect coherence to a pattern

When coherent waves overlap, the locations of constructive and destructive interference remain fixed, producing a stable interference pattern. An ordinary pair of independent light sources usually has a changing phase relationship and does not produce a stable pattern.

Use one source when needed

A single source split into two paths can provide a common frequency and phase relationship. The resulting secondary sources can then act coherently for a double-source interference experiment.

Common trap

Same frequency alone is not enough. The phase difference must also remain constant; otherwise bright and dark locations move or wash out over time.

C.3.6 Exam Analysis

Assessment in practice

1–2 marks
How it is assessed

Questions ask why two sources need to be coherent. The evidence repeatedly rewards constant phase difference and the resulting fixed pattern, often with a single source split into two paths.

Command terms

Explain

What earns marks

State both conditions for coherence: same frequency and constant phase difference. Then link the fixed phase relationship to a stable bright/dark interference pattern, and explain why independent sources usually fail.

Watch for

Mentioning only equal frequency and omitting the requirement that phase difference remains constant.

Representative question

Question 1

[Maximum number: 2]

Explain why the two sources need to be coherent for the interference pattern to be observed.