Interference in Thin Films
Category : JEE Main & Advanced
Interference effects are commonly observed in thin films when their thickness is comparable to wavelength of incident light (If it is too thin as compared to wavelength of light it appears dark and if it is too thick, this will result in uniform illumination of film). Thin layer of oil on water surface and soap bubbles shows various colours in white light due to interference of waves reflected from the two surfaces of the film.
In thin films interference takes place between the waves reflected from it?s two surfaces and waves refracted through it.
(1) Interference in reflected light : Condition of constructive interference (maximum intensity)
\[\Delta =2\mu \,\,t\cos r=(2n-1)\frac{\lambda }{2}\]. For normal incidence r = 0 so \[2\mu \,t=(2n-1)\,_{2}^{\lambda }\]
Condition of destructive interference (minimum intensity)
\[\Delta =2\mu \,t\,\cos r=(2n)\frac{\lambda }{2}\].
For normal incidence \[2\mu \,t=n\lambda \]
(2) Interference in refracted light : Condition of constructive interference (maximum intensity)
\[\Delta =2\mu \,t\,\cos r=(2n)\frac{\lambda }{2}\]. For normal incidence \[2\mu \,t=n\lambda \]
Condition of destructive interference (minimum intensity)
\[\Delta =2\mu \,t\,\cos r=(2n-1)\frac{\lambda }{2}\]
For normal incidence \[2\mu \,t=(2n-1)\frac{\lambda }{2}\]
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