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**Thin-Film Interference: Coating a Lens -- Answers found, explanation needed!**

## Homework Statement

Boxed answers are the correct ones.

## Homework Equations

2L=(m+1/2)(λ/n) for constructive interference (maximum reflection, bright film)

2L=(m)(λ/n) for destructive interference (minimum reflection, dark film)

Where:

L = thickness of film

m = fringe count (for m=0,1,2,3,4...)

λ = wavelength of light

n = refractive index of coating

## The Attempt at a Solution

I got the correct answer (a) for Fall2008 #20 by using the equation for constructive interference, plugging in 0 for m, 1000 nm for λ, and 1.8 for n.

Now for Sp2011 #11, I took it as essentially the same question except this one is asking for minimum reflection, so I should use the equation for destructive interference. Doing that gets me (e), which is incorrect. The correct answer is (b), which is half of (e), implying that I should use the equation for constructive interference.

Am I missing a fundamental difference between the two problems besides one asking for minimum and another asking for maximum reflection? Thanks for the help.

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