What are coherent sources of light – Physics – eSaral

Download India's Best Exam Preparation App

Class 9-10, JEE & NEET

Download eSaral App

Hey, do you want to know what are coherent sources of light? If yes. Then keep reading.

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


  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

  so, that’s all from this article. I hope you get the idea about What are the coherent sources of light. If you found this article informative then please share it with your friends. If you have any confusion related to this topic then feel free to ask in the comments section down below.

For a better understanding of this chapter, please check the detailed notes of the Wave Optics To watch Free Learning Videos on physics by Saransh Gupta sir Install the eSaral App.