Film thickness for strong reflection at 610 nm

[astru025]

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?

Homework Equations

Not sure where to even start. Any help would be nice!

The Attempt at a Solution

Just need to be pointed in the right direction please!

 

[Kishlay]

show us your attempt, then we will help you...!

 

[ehild]

Think of interference on thin films.

ehild