Coherent Sources of light

Coherent sources are those sources of light that emit continuous light waves of the same wavelength, same frequency and are in the same phase or have a constant phase difference.
For observing interference phenomenon coherence of light waves is a must.
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.
Two independent sources of light are not coherent and hence can not produce interference because the light beam is emitted by millions of atoms radiating independently so the phase difference between waves from such sources fluctuates randomly many times per second.
The coherent sources can be obtained either by the source and obtaining its virtual image or by obtaining two virtual images of the same source. This is because any change of phase in the real source will cause a simultaneous and equal change in its image.
Generally, coherence in interference is obtained by two methods
(1) Division of wavefront where wavefront is divided into two parts by reflection, refraction or diffraction and those two parts reunite at a small angle to produce interference as done in case of Young's double-slit experiment and Fresnel's biprism experiment.
(2) Division of amplitude where the amplitude of a section of the wavefront is divided into two parts and reunited later to produce interference such as in case of thin films.
Laser light is almost monochromatic light with little spreading and two independent sources of laser light can produce an observable interference pattern.

1. Thanks for simple explanation! Found it helpful in recalling the concept!

2. nice to know that you like the post ....

3. Nice post...Good work..:-)

4. wat an idea sir jeeeee!!!

5. awesome post....
i liked it

6. is it not sufficient that the sources are of same frequency because i think then they will automatically be coherent

1. Hi please see point number 4 for explanation why two different or independent sources of light can not be coherent .