# Confused on Constructive/Destructive Interference

#### BuBbLeS01

Confused on Constructive/Destructive Interference!!!

1. The problem statement, all variables and given/known data
A film with n=1.6 is deposited on glass. What is the thinnest film that will produce constructive interference in the reflection of light with a wavelength of 575 nm?

2. Relevant equations

3. The attempt at a solution
I have 2 equations that I can choose from...

Constructive Interference for 0 to 2 refl. phase changes
Destructive Interference for 1 refl. phase changes
2T = m * t/n
t = wavelength

Destructive Interference for 0 to 2 refl. phase changes
Constructive Interference for 1 refl. phase changes
2T = (m + 1/2) * t/n
t = wavelength

How do I know which one to use?

I chose the 2nd one because I assumed there was only 1 phase change.

How do figure out what m =???

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#### Snazzy

You can't just assume there's only one phase change. You have to see if the reflected beams of light undergo a hard or soft reflection.

As for the value of m, you want it to be as small as possible so that the thickness is as small as possible.

#### BuBbLeS01

whats hard and soft? those terms are not in my book.

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