Confused on Constructive/Destructive Interference

AI Thread Summary
The discussion centers on determining the thinnest film thickness for constructive interference in light reflection from a film with a refractive index of 1.6 and a wavelength of 575 nm. Participants are confused about which equations to apply for constructive and destructive interference, particularly regarding phase changes during reflection. The choice of equation depends on whether there is a hard or soft reflection, which is not clearly defined in the provided materials. The goal is to minimize the value of m to achieve the smallest possible film thickness. Clarification on reflection types and their impact on phase changes is needed for accurate calculations.
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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 =?
 
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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.
 
whats hard and soft? those terms are not in my book.
 

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