Question

What are coherent sources? How are they realised in practice?

Answer 1

Two sources are supposed to be coherent when the waves released from them have equal frequency and fixed phase differences. In practice, two methods to generate coherent sources: (i) interference of light occurs between waves from the actual source and a practical source (ii) Interference of light occurs between waves from two sources produced due to a single source.

Complete step-by-step solution:
Two waves are supposed to be coherent sources if they transmit light waves of equal frequency and approximately equal amplitude and are constantly in phase with each other. It is challenging to have two sources that appear as coherent sources. Coherent sources have characteristics: The waves produced have a fixed phase difference, and the waves are of an individual frequency.
Two light sources are supposed to be coherent if they release light which always has a fixed phase difference. It suggests that the two sources must release radiations of the equivalent wavelength. The two individualistic sources cannot be coherent because independent sources cannot maintain a fixed phase difference. For experimental goals, two virtual sources taken from a single parent source can perform as coherently. In such a case, all the irregular phase changes occurring in the parent source are replicated in the virtual sources, thus keeping a fixed phase difference between them. Since the wavelength of light waves is tiny, the two sources need to be narrow and close to each other.
In practice, it is impossible to hold two individual sources which are coherent. However, two virtual sources produced by a single source can act as coherent sources for experimental plans. Techniques have been devised where light Interference occurs between waves from the actual source and a virtual source, and Interference of light occurs between waves from two sources produced due to a single source.

Note: There are two ways for achieving coherent sources in the laboratory:
(a) Division of wavefront: In it, the wavefront is split into two or more parts with the advice of mirrors, lenses, and prisms. The standard methods are Fresnel's biprism method, Young's double-slit arrangement, Lloyd's mirror method, etc.
(b) Division of amplitude: In it, the amplitude of the incoming beam is split into two or more components by partial reflection or refraction. These split parts cover different paths and are reunited later to generate interference—Eg, Michelson's interferometer, Newton's rings etc.