The phase relationship between two light waves can very from time to time and from point to poitn in space. The property of difinte phase relationship is called coherence.

Temporal coherence : In a light source a light wave (photon) is produced when an excited atom goes to the ground state and emits light.

(i) The duration of this transition is about 10^-9 to 10^-10 sec.Thus the emitted wave remains sinusoidal for this much time. This time is known as coherence time (τ_c).

(ii) Definite phase relationship is maintained for a length L = cτ_c called coherence length. For neon λ = 6328A⁰,τ_c≡ 10^-10 sec and L = 0.03 m.

            For cadmium λ = 6428A⁰ , τ_c≡ 10^-5  sec and L = 0.3 m.

            For Laser τ_c≡ 10^-5  sec and L = 3 km.

(iii) The spectral lines width Δλ is related to coherence length L and coherence time τ_c .

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