401. Why is a semi-transparent mirror better than metal and how does it work?
Metal mirrors usually absorb about 5% of the light that strikes them. Thus a fully reflective metal mirror, with a thick layer of aluminum, silver, gold, or some other metal, will typically only reflect about 95% of the light. A partially reflective metal mirror, with a very thin layer of metal, might reflect 50% of the light, transmit 45% of the light, and absorb 5%. That 5% absorption is terrible in a laser because the metal layer will heat up and fall apart. Instead, dielectric (insulator) mirrors are created. These mirrors used layer after layer of perfectly clear insulators (usually metal oxides and metal fluorides) to reflect light. Each time light moves from one of these layers to the next, its speed changes and part of it reflects. The thicknesses of the layers are carefully controlled so that the desired wavelengths are reflected in just the right amounts. Since the layers absorb no light, any light that is not reflected is transmitted. A dielectric mirror might reflect 50% of the light, transmit 50% of the light, and absorb 0%. Since they absorb no light, dielectric mirrors do not heat up in use and work well with even very high-powered lasers.