Interference of light - flat glass and an air wedge

[glass.shards]

Homework Statement

A fine hair separates one end of two pieces of flat glass to form and air wedge.when light of wavelength 670 nm is incident normally, 25 dark bands are observed (with one at each end).How thick is the hair?

Homework Equations

destructive interference: 2nt = (m+1/2)lambda

The Attempt at a Solution

I really have no idea how to solve this problem, or even what a diagram would look like. Is the glass positioned like a tipi? Where is the light hitting the glass, and where are the bands even being observed?

Thanks in advance!

 

[mgb_phys]

The gap between the two pieces of glass forms a triangle
Light bouncing between the glass surfaces at the top and bottom of the air gap will interfere either constructively or destructively to give a bright or dark stripe depending on if there is a whole number of half wave-lengths.