C.3.6—Coherence
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
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
[ 2 ]
Explain why the two sources need to be coherent for the interference pattern to be observed.
Retrieve the Core C.3 Wave Phenomena Model
C.3 Wave phenomena is secure when you can connect the physical picture to the equation and its limits.
- Wavefronts and rays
- Reflection, refraction and Snell’s law
- Refractive index and total internal reflection
- Superposition, coherent sources and interference
- Young’s double-slit pattern