Confused on Constructive/Destructive Interference

[BuBbLeS01]

Confused on Constructive/Destructive Interference!

Homework Statement

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?

Homework Equations

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 =?

 

[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.