Question

what is coherent source?

Answer 1

Two sources are said to be coherent if they produce waves
of same frequency with a constant phase difference. Unlike
sound waves, two independent sources of light cannot be
coherent. Since sound is a bulk property of matter, therefore,
two independent sources of sound can be identical
respects and can produce coherent waves. On the contrary,
light is not a bulk property of matter, it is a property of each
individual atom. As the individual atoms emit light randomly
and independently, therefore, two independent sources of light
cannot be coherent.
Coherent sources can be obtained by splitting a light beam
from a source into two. This can be done in two ways:
1. Division of Wavefront: It is done in Young's double-slit
experiment, Fresnel's biprism, Lloyd's mirror, etc.
2. Division of Amplitude: It is usually done by partial
reflection and transmission at a boundary, as it occurs in
thin film interference, Newton's rings, etc.

Answer 2

Hope it helps ..........