Question

What is longitudinal cohherence?

Answer 1

Longitudinal Coherence Definition: Distance over which two waves from the same source point with slightly different wavelengths will completely dephase. λ λ−∆λ P 2LL Two waves of slightly different wavelengths λ and λ − ∆λ are emitted from the same point in space simultaneously. After a distance LL, the two waves will be exactly out of phase and after 2LL they will once again be in phase. 2LL = Nλ 2LL = (N + 1)(λ − ∆λ) Nλ = Nλ + λ − N∆λ − ∆λ 0 = λ − N∆λ − ∆λ −→ λ = (N + 1)∆λ −→ N ≈ λ ∆λ −→ LL = λ 2 2∆λ