In the previous section we described why lenses are coated, and in this section we continue with why coatings increase light transmission.
Transmittance enhancement film is based on the fluctuation of light and interference phenomenon. If two light waves of the same amplitude and wavelength are superimposed, the amplitude of the light waves is enhanced; if the amplitude is the same and the wavelength is opposite, the two light waves superimposed cancel each other out. Reflective film is the use of this principle, in the surface of the lens coated with reflective film, so that the reflected light generated by the front and rear surface of the film layer interferes with each other, thus canceling the reflected light, to achieve the effect of reflective reduction.
1) Amplitude condition
The refractive index of the film layer material must be equal to the square root of the refractive index of the lens sheet base material.
The thickness of the film layer should be 1/4 of the wavelength of the reference light. d=λ/4 λ=555nm, d=555/4=139nm
For example, the wavelength of light wave with high sensitivity of human eye is 555nm, the thickness of film layer should be 555/4=139nm. When the thickness of coating is too thin less than 139nm, the reflected light will show light brownish yellow, if it is blue, the thickness of coating is too thick more than 139nm.
The purpose of the reflective coating is to reduce the reflection of light, but it is not possible to do so without reflecting light. The surface of the lens will always have a residual color, but the residual color which is the best, in fact, there is no standard, more green. That is why we often see telescope lenses with green reflections.
We also see a green color in the central part of the lens and a light fuchsia or other color in the edge part, because the curvature of the lens is different between the convex and concave surfaces, which also makes the coating reflective color different.