Question on thin film interference

[crosbykins]

Homework Statement

A piece of paper of thickness 15 * 10^-6 m is used to create an air wedge between 2 glass slides that are 7.25 cm long. If the wavelength of the light being used is 595 nm, how many interference fringes are counted across the entire pattern?

Homework Equations

delta x = L ( wavelength/2t)

The Attempt at a Solution

delta x = (.0725)[(595 * 10^-9)/(2)(15 * 10^-6)]
= 1.44*10^-3 m

*now i know the distance between the dark fringes so i can divide the length of the glass slides to find the number of fringes:

.0725/1.44*10^-3
= 50 fringes

***is this correct?*** if it's wrong, a step-by-step solution would be greatly appreciated

 

[eczeno]

looks good to me. although i think it would be 51, because the pattern starts with a dark fringe, and then proceeds 50 dx's down the road before the last dark fringe. if you look at it like a fence, it takes 51 fence posts to hang 50 lengths of fence. the dx's are the lengths of fence and the dark fringes are the posts.

cheers