Question

Two sources are called coherent if
(A) Both have same amplitude of vibration
(B) Both produce waves of the same wavelength
(C) Both produce waves of the same wavelength having constant phase difference
(D) Both produce waves having the same velocity

Answer 1

Hint: Firstly, coherence is an ideal property of waves that enables stationary interfaces and two waves are said to be coherent if they have a constant relative phase. This source is created if the amplitude of an incoming beam of light is divided or the wavefront gets divided.

Complete answer:
Coherent sources: If the sources which have zero or constant phase difference have the same frequency or we can say wavelength, then the two sources are considered to be coherent. The coherence of two waves defines how well two waves are correlated with each other.
Both option (B) and (C) are seems to be correct but the waves will be considered to be cohearts only when it has constant phase difference also.
Therefore the correct option is (C).

Additional information:
There are two method of producing coherent source:
Division of wavefront which is possible with the help of mirrors, lenses and prisms.
Division of amplitude where division takes place by partial reflection or refraction.
There are two types of Coherence:
Temporal Coherence – measure how monochromatic a source is.
Spatial Coherence – describes the ability of two points in space of a wave that interfere.

Note:
Remember that the laser light and sound waves produce coherent waves as they produce waves of same frequency as well as with same phase. And also it is important to know not only two waves can show coherent property, more than two waves can also show coherent property.