Fringe Radius Relation for Small Times t

[Ciato]

1. Show that for t<<R, the radius r of a fringe is related to r= (2TR)^1/2

I'm not sure what the relevant equations are, that would be the problem. A gentle shove in the right direction would be appreciated, I've been looking for exactly how to prove it and I can't. ^_^ Thanks for any help!

 

[Doc Al]

This relationship is true in general, not just for fringes. Examine the geometry of the Newton's rings set up: A spherical convex lens placed upon a flat piece of glass. Draw a diagram with the quantities r, t, and R indicated.