Interference of light wave is the modification in distribution of light energy obtained by superposition of two or more waves
At some points where crest of the one wave falls on the crest of another ,resultant amplitude is maximum
At some points where crest of one wave falls on trough of another, the resultant amplitude become minimum and hence intensity of the light is minimum
At points, where the resultant intensity of light is maximum ,the interference is said to be constructive
At points where resultant intensity of light is minimum ,interference is said to be destructive
7) Coherent Sources
Coherent sources are those sources of light which emit continuous light waves of same wavelength ,same frequently and are in same phase or have a constant phase difference
For observing interference phenomenon ,coherence of 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 cannot produce interference because light beam is emitted by millions of atoms radiating independently so that phase difference between waves from such fluctuates randomly many times per second
Two 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 in phase in real source will cause a simultaneous and equal change it its image
Generally coherence in interference is obtained by two methods i) Division of wave front where wave front is divided into two parts by reflection ,refraction or diffraction and those two parts reunite at a small angle to produce interference such as in case of Young Double slit experiment ,Fresnel bi-prism . ii) Division of amplitude whose amplitude of a section of wave front is divided into two parts and reunited later to produce interference such as in case of interference due to thin films
Laser light is almost monochromatic with light spreading and two independent laser sources can produce observable interference pattern
8) Conditions for sustained interference of light waves
Two sources should continuously emit waves of same wavelength or frequency
The amplitudes of the two interfering waves should be equal or approximately equal in order to reduce general illumination
The sources of light must be coherent sources
Two sources should be very narrow as a broad source is equivalent to large number of narrow sources lying side by side which causes loss of interference pattern resulting general illumination
Two sources emitting set of interfering beams must be placed very close to each other so that wavelength interact at very small angles
