Question

Two sources are called coherent if they produce waves
(a) of equal wavelength
(b) of equal velocity
(c) having same shape of wave front
(d) having a constant phase difference

Answer 1

(d) having a constant phase difference
For light waves emitted by two sources of light to remain coherent, the initial phase difference between waves should remain constant in time. If the phase difference changes continuously or randomly with time, then the sources are incoherent.