How to Derive the Formula for Separation in Fresnel's Biprism Experiment?

[ProPatto16]

Homework Statement

Show that the separation of the two coherent sources is given by d=2aA(n-1) where n is the refactive index of prism.

Homework Equations

from youngs double slit experiment y=Rm$\lambda$/d

The Attempt at a Solution

i have no idea how to derive this formula. I've seen it many times before but nowhere shows the derivation. i can see youngs experiment is similar, but fail to see a connection?

help!

there should be a picture attached to this post. hopefully!

 

[ehild]

Try this URL:http://discovery.bits-pilani.ac.in/~raj/fresnel-biprism.pdf

ehild