Coherent vs. Incoherent Light

∟Coherent vs. Incoherent Light

This section describes differences between coherent light and incoherent light.

As described in the previous section, photons released from a laser generator are generated from the stimulated emission process. Those photons have identical properties: wavelength, direction, phase, and polarization. They also travel in phase.

Coherent Light - Light formed by photons with identical wavelength, direction, phase, and polarization is called coherent light. A laser is a good example of coherent light.

Photons in coherent light are superposed together to form a continuous uniform electromagnetic wave as shown below (magnetic waves are omitted):

Coherent Light Formed by Identical Photons

From a wavefront perspective, we can have 3 categories of coherent light:

Single-point coherent light - An idealized laser generator releases identical photons from a single point. And all photons travel in a single line. The wavefront of the laser beam is maintained as a single point.
Non-diverging coherent light - A less ideal laser generator releases identical photons from a small opening. And all photons travel in lines parallel to the center line. The wavefront of the laser beam is maintained as a fixed small flat circle. The this type electromagnetic wave is considered as plane wave or planar wave.
Diverging coherent light - A real laser generator releases identical photons from a small opening. Photons spread slowly away from the center line. The wavefront of the laser beam starts as a fixed small flat circle, and becomes a large spherical surface over time. The divergence rate of a good laser generator is about 1 milliradians (1 millimeter over 1 meter, or 1 meter over 1,000 meters)

Wavefront of Diverging Coherent Light (edmundoptics.com)

Incoherent Light - Light formed by photons with different properties is called incoherent light. Sun light is a good example of incoherent light.

In an incoherent light beam, photons with identical properties: wavelength, phase, and polarization, form individual continuous electromagnetic waves. Multiple continuous electromagnetic waves with different properties superposed together to form a single overall electromagnetic wave.