It simply consists of two partially reflecting mirrors, placed parallel to each other. This might be the basis for an optical transistor. If a beam of light strikes the first mirror, some percentage of the light is reflected, and some goes through. The same happens at the other mirror. Now we are going to use the latter.
Waves interfere under certain circumstances .
If the mirrors are placed at a distance equal to halfway an integral number of half wavelengths, the beams of light that are in the cavity interfere constructively. This means that the transmitted beam is far more than 1 percent, it might even be a 100 percent. On the other hand, if the mirrors are placed at an integral number of half wavelengths, the waves interfere destructively. The transmitted beam is even less than 1 percent. Thus, we have some sort of a switch.