We already know that the transmittance is related to the refractive index of the coating, but not about its thickness. However, if we can work hard on the thickness of the coating, we will find that the difference between reflected light a and reflected light B is NC × Optical path difference of 2D. If
nc × 2D=（N+ 1/2） λ Where n = 0,1,2,3,4,5 λ Is the wavelength of light in the air
It will cause the reflected light of this specific wavelength to have the effect of cancellation, so the color of the reflected light will change.
Optical coating machine
Optical coating machine
For example, if the thickness of the coating causes the cancellation of green light, the reflected light will appear red. Many telescopes on the market that look like red lenses are made based on this principle. Nevertheless, the transmitted light does not turn red.
In many complex optical systems, the suppression of reflected light is a very important lesson. Therefore, between a group of lenses, different coating thickness will be used to eliminate the reflected light of different frequencies. Therefore, the more advanced the optical system, the more colors of reflected light will be found.
