Also known as the photoemissive effect, it involves the emission of electrons
from a surface when that surface is struck by electromagnetic radiation,
such as x-rays. No electrons are emitted unless the energy of the incident
photons exceeds a certain threshold value known as the surface work function
for photoemission from the material. The effect is used in phototubes
that have photoemissive cathodes, "electric eyes," and television
cameras, to name a few applications.