What is the longest wavelength λ of light that will exhibit constructive interference at normal incidence? the keratin layers are thin enough that you can think of them as representing the surfaces of a 200-nm "film" of air?

560nm

Explanation:

You can use the following expression for thin films:

where n is the index of refraction, d is the thicknes of the thin film and m is the order of the diffraction pattern. For keratin layers we can use n=1.40. For the longest wavelenth we use m =1.

Hence, we obtain:

HOPE THIS HELPS!!

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.