Why are there so many colors?
This has to be analyzed for the purpose of the coating.
Both glasses and lenses want to transmit as much light as possible at the required wavelengths (that is, light of certain colors).
However, when light enters different transmission substances (such as air into glass), about 5% will be reflected off, and there are many lenses and refractors in the optical sight, which can add up to 30% to 40% loss of incident light.
These films are called “antireflective films” and, as their name implies, can improve the transmission of light at certain wavelengths.
Next, let’s talk about why the film can increase transmission.
Light is vertically incident from the medium n1 into n2, and its transmittance=4n1n2/(n1n2)².
It is assumed that the refractive index of air n=1, glass and film are n1=1.8, Reflective rate n2=1.5
Before coating, the transmittance =4*1.0*1.8/(11.8)²=0.9184
After coating, the transmittance =[4*1.0*1.5/(11.5)²]*[4*1.5*1.8/(1.518)²]=0.952
There is indeed an improvement.
In detail, the color of each coating mainly depends on the thickness.
Lenses plated with a single layer of antireflective film are usually blue-purple or red, while lenses plated with multi-layer anti reflective film are light green or dark purple.
One of the purposes of multilayer coating is to use the above formula to further improve the transmittance, and the other purpose is not only to increase the transmittance, such as the following picture is the coating on the resin glasses, the coating layer on the lens from bottom to top is the lens with hard film, connecting layer, multi-layer anti reflective film, water-repellent film.