Coherent Source of Light : "The coherent sources of light are those sources which emit light waves of the same wavelength, same frequency and in same phase or having constant phase difference."
But two independent sources cannot be coherent because every source of light is made up of innumerable atoms which are the ultimate source of light. At one instant, certain atoms emit light while at the other instant certain other atoms emit light. This process continues. Therefore, the light wave emitted from a source is not a continuous wave, but is made up of pulse-waves emitted from a source. Since each atom of the source emits a pulse-wave independently, the initial phase of the resultant wave coming from the source depends only upon chance. Hence the initial phase of the wave varies with time. Evidently, if we take two independent light-sources, then the phases of the waves emitted by them will vary independently of each other, that is, the phase difference between them will not remain steady and the wave will not form a sustained interference pattern.
If we adopt some device to derive two sources from a single original source, then any phase-change in one will simultaneously be accompanied by the same phase-change in the other. Thus, the phase difference between the two sources will remain constant. Such sources are 'coherent sources' and can give a steady interference pattern.
There are two general methods for producing coherent source :
(i) By Division of Wavefront : In this method wavefront is divided into two or more parts by use of mirrors, lenses or prisms. For example, Young's double slit method, Fresnel's biprism, Lloyd's mirror method.
(ii) By Division of Amplitude : In this method the amplitude of incoming beam is divided into two or more parts by partial reflection and refraction, as in the phenomenon of brilliant colours in thin films of transparent material like soap film, oil film etc.
Recently coherent sources of visible light have been developed whose light is highly coherent. These are called 'Lasers'. Their light is monochromatic and very intense.
