Optics finding index of refraction

[Liquidxlax]

Homework Statement

In order to determine the refractive index of a transparent plate of glass, a microscope is first focussed on a tiny scratch on the upper surface and the barrel position is recorded. Upon further lowering the microscope barrel by 1.87mm , a focussed image of the scratch is seen again. The plate thickness is 1.5mm. What is the reason for the second image and what is the refractive index of the glass?

Homework Equations

nglass/nair = y/y'

The Attempt at a Solution

i know the reason for the second image, my problem is I'm not sure what to do with the data given, because all that I've tried my answers are too high or to low. I'm also not sure if that equation is relevant, or what to use. I've read my text a few times, and i see nothing that even relates to this problem.

 

[Liquidxlax]

does anyone have an idea?