Unclear geometry in optics problem

[ggb123]

Homework Statement

[/B]
A parallel quadratic slab of glass (n = 1.55 and thickness d = 2 cm, L = 21 cm) rests on a large slab of glass (n = 1.55). To prevent the optical contact weld forming between the two polished surfaces, a small teflon ball (D = 1 cm) is inserted between the slabs on one side 1 cm away from the edge. 500 nm light is incident perpendicularly on this structure.

(a) Find the formula for the dark fringe position as the function of the distance from the point of contact.

(b) Find the number of bright fringes formed.

Homework Equations

The Attempt at a Solution

This question should be simple enough as soon as I get a clear picture of the geometry of the system. Can anybody provide some insight?

 

[gneill]

It sounds like something like this:

The only grey area I see is how to interpret "500 nm light is incident perpendicularly on this structure". Does it mean perpendicular to the large glass slab (so the light is vertical), or to the top slab which is at slight angle thanks to the teflon ball? I suspect the latter.