IB Physics SL C.3.6 Coherence

Practise identifying coherent sources by equal frequency and constant phase difference and explaining why stable interference fringes disappear when phase varies randomly.

Syllabus
First assessment 2025
Objective
—
Level
SL

Exam points

  • state that coherent sources have the same frequency and a phase difference constant with time
  • distinguish coherence from equal amplitude, equal intensity or simply originating near one another
  • explain stable fringes through fixed phase and washed-out patterns through random phase changes

C.3.6—Coherence question 1

[Maximum number: 1]

This question is about interference.

Light from a laser is incident on two identical parallel slits. The light from the two slits produces a fringe pattern on a screen.

Figure for Question C.3.6—Coherence question 1 — IB Physics SL

A central bright fringe is produced at C . The next bright fringe is produced at A . There is a dark fringe at B .

The light from the laser is coherent and monochromatic. Outline what is meant by the term

coherent.

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