Light is full of energy; it’s just that we can’t do much with that energy directly. Whether we’re trying to use the energy to power our world or simply to detect the light itself, we have to extract it. This typically means converting the light to some other more useful form of energy.
There are two fundamental kinds of light conversion: ones that turn light into other light and ones that turn light into electric current or voltage.
The first category is referred to generically as luminescence. There are two kinds of luminescence, which depend on the light and material. When light is absorbed and immediately re-emitted at a different frequency, it’s called fluorescence – as seen in a fluorescent lightbulb. When the light is re-emitted more slowly over time due to illegal quantum state interactions of some sort that slow the process down, it’s called phosphorescence. The phosphorescent delay is familiar in glow-in-the-dark materials, where the light is absorbed and then re-emitted gradually.
When fluorescence involves ionizing radiation, like X-rays, which have enough energy to knock electrons out of an atom, then the resulting re-emission (in the visible range) is referred to as scintillation.
When converting light to electrical voltage or current, there are also several mechanisms. A very simple phenomenon is photoconductivity, where the conductivity of a material (in particular, a semiconductor) is improved because the energy of incident light is bumping more electrons into the conduction band.
There are two other closely-related effects that can result in electrons being knocked about. In the photovoltaic effect, electrons are transferred between bands, creating a potential between electrodes. With the photoelectric effect, the electrons are completely ejected from the atom and are free to roam about the cabin.