an optical cavity is "an arrangement of mirrors that forms a standing wave cavity resonator for light waves" (wikipedia).
The possible standing wave patterns for such structure are like these:
As you can see, the vertical black lines (which are the mirrors) are the nodes of the standing waves, since they force the wave to be 0 at those points.
Well, I have studied a similar situation for electromagnetic resonant cavities. In such devices, the mirrors were replaced by walls made of perfect electric conductors, and these faces were the nodes of the standing waves because they forced the tangential electric field to be 0 along them (which is the interface condition for perfect electric conductors). But in this case, the walls are generic mirrors, so I do not understand why they force the wave to be 0 along them.
So my question is: why do the mirrors force the wave to assume always 0 amplitude, i.e. mirrors are the nodes of the standing waves?