Question

What is optical path? Obtain the equation for optical path of a medium of thickness d and refractive index n.

Answer 1

1. Optical path of a medium is defined as the distance d’ light travels in vacuum at the same time it travels a distance ‘d’ in the medium.
2. n = refractive index.
3. d = thickness
   ν = speed of light in the medium

v = `"d"/"t" ("or") "t" = "d"/"v"`

c = `("d"')/"t"` or t = `("d"')/"c"`

as time is same

`("d"')/"c" = "d"/"v"`

d' = `"c"/"v"`d

d' = nd

∵ n = `"c"/"v"`

As ‘n’ is always greater than 1, the optical path d’ of the medium is always greater than 1.