The distance traversed by light in a medium of refractive index m in time t is given by d=vt .......(i) where v is velocity of light in the medium. The distance traversed by light in a vacuum in this time, Δct=c×
d
v
[from equation (i) ] =d
c
v
=µd ......(ii) (∵µ=
c
v
) This distance is the equvalent distance in vacuum and is called optical path. Optical path for first ray which travels a path L1 through a medium of refractive index n1=n1L1 Optical path for second ray which travels a path L2 through a medium of refractive index n2=n2L2 Path difference =n1L1−n2L2 Now, phase difference =