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Interference

01Interference

Coherence & path difference

This section covers coherence, path difference and how the routes travelled by two waves determine their relative phase when they meet.

Coherent waves

Two sources are coherent if they oscillate at the same frequency while their phase relationship remains constant. This fixed relationship allows a stable interference pattern to form.

A laser is useful in light-interference experiments because it supplies light that is both coherent and monochromatic. Monochromatic light has a single wavelength.

Path difference

Path difference compares the lengths of the two routes followed by waves from their sources to the same observation point.

For coherent sources S1S_1 and S2S_2 meeting at PP:

path difference=∣S2P−S1P∣\text{path difference}=|S_2P-S_1P|

Path difference is usually expressed in terms of the wavelength λ\lambda. Its value determines the phase relationship of the two waves when they reach PP.

S1S2PS1PS2PjS2P¡S1Pj

Path difference concerns distance travelled. Phase difference instead compares the positions of the waves within their oscillation cycles.

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