Solving a problem in physics section 42 point 16 optics

16. In order to reduce light loss due to reflection from the glass surface is carried out, "the enlightenment of optics": the free surface of the lens is applied to a thin film with a refractive index n = √nc. In this case, the amplitude of the reflected waves from the both surfaces of a film is the same. Determine the thickness of the layer in which the reflection for light with a wavelength λ of glass in the normal direction is zero.