Thin Films again - Help?

[rachiebaby17]

Homework Statement

A camera lens (n = 1.90) is coated with a thin film of magnesium fluoride (n = 1.40) of thickness 90.0 nm. What wavelength in the visible spectrum is most strongly transmitted through the film?

Homework Equations

?

The Attempt at a Solution

I have no idea where to start, is there an equation that will use both n's at once?

 

[rl.bhat]

Search in Google for thin film interference.