Jawaharlal Nehru Technological University Hyderabad B.Tech I Year ENGINEERING PHYSICS I Previous Question Paper (September/October - 2021 - R16)

The 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. Common examples include Young's double-slit experiment, Fresnel's biprism, and Lloyd's mirror.

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

The optical path difference between the two reflected rays is given by:

where r is the angle of refraction. Due to the reflection at the denser medium (top surface), an additional phase shift of π occurs, equivalent to an optical path difference of λ/2. Thus, the effective path difference is:

Constructive interference (bright fringe) occurs when Δ = nλ, leading to the condition 2μt cos r = (2n + 1)λ/2. Destructive interference (dark fringe) occurs when Δ = (2n + 1)λ/2, leading to 2μt cos r = nλ.