# Very hard physics problem!

#### astru025

1. Homework Statement

You are coating a glass lens of index of refraction 1.6 with a film of material of index of refraction 1.7. You start with the thinnest film possible that creates a strong reflection for 610 nm light. You gradually increase the film thickness until you again get strong reflection. What is the thickness of the film now?

2. Homework Equations

Wavelength in film = wavelength in a vacuum / index of refraction of film

610 nm / 1.6 = my answer? Not sure if this is correct.

3. The Attempt at a Solution
Just need to be pointed in the right direction! Really am not sure where to start for this problem. Any help would be really nice!

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#### vela

Staff Emeritus
Homework Helper
1. Homework Statement

You are coating a glass lens of index of refraction 1.6 with a film of material of index of refraction 1.7. You start with the thinnest film possible that creates a strong reflection for 610 nm light. You gradually increase the film thickness until you again get strong reflection. What is the thickness of the film now?

2. Homework Equations

Wavelength in film = wavelength in a vacuum / index of refraction of film

610 nm / 1.6 = my answer? Not sure if this is correct.
Probably not.

3. The Attempt at a Solution
Just need to be pointed in the right direction! Really am not sure where to start for this problem. Any help would be really nice!
It's fairly obvious you're just guessing. I mean, how do you know the answer isn't 610 nm/1.7? Or maybe it's some combination of 1.6 and 1.7. Or maybe it has absolutely nothing to do with the indexes of refraction. Why do you think it does? (It could be the information is superfluous.)

Think about what's going on here. What's happening when get a strong reflection?

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