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## Homework Statement

Monochromatic light of wavelength 500nm shines on a pair of identical glass microscope slides that form a very narrow wedge, as shown in the diagram. The top surface of the upper slide and the bottom surface of the lower slide have special coatings on them so that they do not reflect light. The inner surfaces are reflective. The indices of refraction of air and glass are 1.00 and 1.50 respectively. Looking down on the slide from above, you see the fringe pattern shown.

http://img519.imageshack.us/img519/1517/picture2dt0.png [Broken]

## Homework Equations

[tex]\lambda_{D}[/tex]=[tex]\frac{2nd}{m-0.5}[/tex]

## The Attempt at a Solution

Since the "P" points to the 4th dark fringe,

m=4

[tex]\lambda_{D}[/tex]=500nm

n=1 (as the light is traveling the distance "d" in air)

d=?

Isolating for d and solving,

My answer is 875nm.

However, the correct answer is 750nm.

If anyone could help me out I'd really appreciate it.

Thanks in advanced.

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