Two sources are coherent if they produce waves with the same frequency and maintain a constant phase difference. Their amplitudes do not have to be equal, although similar amplitudes produce clearer interference patterns.
This is part of C.3 Wave phenomena and applies to both SL and HL students.
The Reasoning
The phase difference describes how far one wave is ahead of or behind another within an oscillation. For coherent sources, this difference remains constant:
The constant does not have to be zero. Two sources can be coherent when they are always in phase, always in antiphase, or separated by any other fixed phase angle.
The sources must also have the same frequency. If their frequencies differ, their relative phase changes continuously, so positions of constructive and destructive interference move with time instead of forming a stable pattern.
| Condition | Why it is required |
|---|---|
| Same frequency | Ensures the phase relationship does not drift continuously with time |
| Constant phase difference | Keeps constructive and destructive interference at fixed positions |