Sun February 17, 2013 By:

What are coherent source ? why are coherent sources required to prouduce sustained interference of light ? In young's double slit experiment , to produce interference patterns due to a monocromatic sources of light , deduce the expression for the fringe width? Draw the intensity curve for interference in this expreiment.

Expert Reply
Sun February 17, 2013
Coherent Sorces of light are the sources which have a phase difference between their wavefronts which do not vary with time and, have same wavelength.
It is required to obtain sustainable interferene pattern because, if the phase difference is not constant and it varies with time, then the pattern will be an average over the time and wont give fringes.
let a system be:
now for a bright fringe at P:
Such bands are called fringes, they are equally spaced and distance between them is called fringe width and is given by:
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