In optics, you have to create a layer of coating that is approximately 1/4 of the light's wavelength. The working equation for this problem is:
d = λ₀/4n,
where
λ₀ is the wavelength of the incident light
n is the refractive index of the coating
Substituting the values,
d = (650 nm)/4(1.39)
d = 116.9 nm
