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The phase relationship between two light waves can vary from time to time and from point to point in space. The property of definite 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.
  • The duration of this transition is about 10–9 to 10–10 s. Thus, the emitted wave remains sinusoidal for this much time. This time is known as coherence time (τc).
  • Definite phase relationship is maintained for a length L = cτc called coherence length. For neon, λ = 6328 Å, τc ≈ 10–10 s and L = 0.03 m.

Methods of obtaining coherent sources

Two coherent sources are produced from a single source of light by two methods (i) by division of wavefront and (ii) by division of amplitude
Division of wave front The wavefront emitted by a narrow source is divided in two parts by reflection, refraction, or diffraction.
The coherent sources so obtained are imaginary. These are produced in Fresnel’s biprism, Llyod’s mirror Youngs, double slit, etc.
Fig. 7
Division of amplitude In this arrangement light wave is partly reflected (50%) and partly transmitted (50%) to produced two light rays.
The amplitude of wave emitted by an extend source of light is divided in two parts by partial reflection and partial refraction.
The coherent sources obtained are real and are obtained in Newton’s rings, Michelson’s interferrometer, colors in thin films.
Fig. 8

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