Two sources of light are said to be coherent when both give out light waves of the same
Wavelength and constant phase difference
Coherent sources:
Where, is the speed of the light, is the frequency, and is the wavelength.
3. Since the speed of the light is constant, and the frequency of the coherent sources is also constant, the wavelength of the sources must be the same.
Hence, the light waves must have the same wavelength and constant phase difference in order to be coherent.
Therefore, option (B) is correct.