Stealth aircraft coating thickness

[Galahad43]

Homework Statement

Stealth aircraft are designed to not reflect radar, whose wavelength is typically around 3.0 cm, by using an antireflecting coating. Ignoring any change in wavelength in the coating, estimate its thickness.
____cm

Homework Equations

Well, that's just the issue. I haven't the foggiest idea how to solve this
Maybe something to do with 1/lambda^4?

The Attempt at a Solution

Searched both the book and internet for the relevant equation, and have not been able to find it. Tried just plain 1/lambda^3, didn't work

 

[rl.bhat]

The antireflecting coating film should by such that signals reflected form top and bosom surface will be out of phase, i.e. the path difference should be λ/2.
Now find the thickness of the film.