JNTU B.Tech I Year ENGINEERING PHYSICS - I Previous Question Paper (September/October - 2021 - R16)

Division of wave front is a method of producing coherent sources by splitting the primary wave front originating from a single monochromatic source into two or more parts. Since these parts originate from the same wave front, they maintain a constant phase relationship, which is a necessary condition for interference. Examples include Young's double-slit experiment, Fresnel's biprism, and Lloyd's mirror, where the wave front is divided by spatial obstruction or refraction.

When a monochromatic light beam of wavelength λ is incident on a thin transparent film of thickness t and refractive index μ at an angle of incidence i, the light undergoes multiple internal reflections at the top and bottom surfaces. The rays reflected from the top surface and those transmitted, reflected from the bottom, and re-emerging from the top interfere with each other.

The path difference between two consecutive reflected rays is given by:

where r is the angle of refraction. Due to the reflection at the rarer-to-denser interface, a phase change of π occurs, introducing an additional path difference of λ/2. The condition for constructive interference (bright fringe) is 2μt cos(r) = (n + 1/2)λ, and for destructive interference (dark fringe) is 2μt cos(r) = nλ, where n is an integer.