Question

Type-B optical flat is tested for parallelism. What is the formula for the difference of the thickness of the flat at 2 points on the adjacent fringe? (µ is the refractive index)

(a) λ/2*(1/µ) microns

(b) λ/4*(µ) microns

(c) λ*µ microns

(d) λ*(µ/2) microns

This question was posed to me by my school principal while I was bunking the class.

This intriguing question comes from Optical Flats in portion Measurement by Light-Wave Interference of Engineering Metrology

Answer 1

Right answer is (a) λ/2*(1/µ) microns

The best explanation: λ/2*(1/µ) microns, this is the formula for the difference of the thickness of the flat at two points on the adjacent fringes. This is obtained by Fizeau interferometer test. This gives the optical parallelism. This parallelism is also represented as geometrical parallelism as flat is made of material of uniform refractive index.