Photosensitive glass reacts to ultraviolet (UV) light much the same way that traditional camera film responds to visible light. Photosensitive glass was invented by Corning's Dr. Donald Stookey in the 1940's.
The most common process for using photosensitive glass use an image negative (like a stencil) to control glass exposure. Both the exposure and the "fixing" of the image (with heat) are achieved over the course of many hours.
The final image is permanent and can vary in both color and opacity depending on exposure and fixing time and temperatures. As the glass becomes increasingly opaque it also will be more vulnerable to hydrofloric acid, expanding the creative possibilities.
As with film, the behavior of photosensitive glass is the result of silver (or gold) ions reacting to light.
Photosensitive glass is different from photochromatic glass, which is used in self-darkening sunglasses. Photochromatic glass darkens relatively quickly when exposed to light but returns to clear when the light source is removed.