Wave Optics - Interference Fringes
Description:
When two coherent sources of light emit light waves in a medium they interfere with each other and form alternate dark and bright bands on the screen which are known interference fringes.
This redistribution of intensity forms a pattern which is called INTERFERENCE PATTERN.
To study interference, we use the following apparatus given by Huygens.
The light emitted from the source is passed by through a cardboard with two closely placed slits in it, these slits acts as two coherent sources of light.
Each source produces its wavefront that propagates towards the observing screen. The phenomenon of superposition of the waves take place resulting in variation in intensity of light at different locations on screen.
When constructive interference occurs it results in a bright band on the screen, conversely when a destructive superposition occurs it results in a dark band on the screen. These are fringes.
There is a formation of such alternate bright (increased intensity) and dark (zero intensity) band on the screen which make an interference pattern.
Consider, Intensity of individual light from each source is ‘a’ i.e. I1 = I2 = a, then
Intensity of bright band = 4a2
Intensity of dark band = 02
This concept was given by Huygen but he was not able to prove it practically, which was latter done by Young.