Interference of light

Young's double-slit experiment :

Path and Optical Path : 

When light travels through a medium of refractive index n and thickness t, the optical path length for that medium is n t. The increase in optical path length is  n t – t = (n – 1)t.

Reflection of waves

If the waves are reflected at a denser medium (i.e. from air to glass) the reflected

waves have a phase change of 180^o ( or p radians ) compared to the incident

waves.

If the waves are reflected at an optically rare dense medium,  no phase change

upon reflection.

Wedge Fringes :

Non-reflecting surface ( blooming of lenses ) :

where     is the thickness of the film and

 λ₁  is the wavelength of light in the film.

(  λ₁ = λ/n  and λ is the wavelength of light in air.)