What are coherent sources of light – Physics – eSaral

Class 9-10, JEE & NEET

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## Coherent source

The two sources of light, whose frequencies are the same and the phase difference between the waves emitted by which remains constant with respect to time are defined as coherent sources. There are two independent concepts of coherence namely

1. Temporal coherence
2. spatial coherence

## Temporal coherence:

In a typical light source (like sodium lamp) a light wave (photon) is produced when an excited atom goes to the ground state and emits light the duration of this $\sim 10^{-9}$ to $10^{-10} \mathrm{sec}$. Thus the electromagnetic (light) wave remains sinusoidal for this much time. This time is known as coherence time it is denoted by $\tau_{c}$.

## Spatial coherence:

Two waves at a different points in space are said to be space coherent if they preserve a constant phase difference over any time t. Note: Laser is a source of monochromatic light waves of a high degree of coherence. The entire wavefront of the laser beam is spatially coherent. Important points: –

1. They are obtained from the same single source.
2. Their state of polarization is the same

Note:

1. Laser light is highly coherent & monochromatic
2. The light emitted by two independent sources (candles, bulbs, etc.) is non-coherent and interference phenomenon cannot be produced by such two sources

(a) Division of wavefront

(b) Division of Amplitude

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