Question

A hair was used to form one end of an air wedge with two glass plates. The glass plates were in contact at the other end. When light of wavelength (4.6x10^-7m) was reflected from the air wedge, the 320th dark
fringe coincided with the position of the hair. What was the thickness of the hair?

Answer 1

http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Interference/text/Wedge_fringes/index.html

So let thickness be e, and the distance from the wedge be x. With the small angle assumption
e =Theta=m*wavelength/2 . In this case, m=320
so e= 320*4.6e-7 /2