What happens when light knocks electrons out of a metal surface? The process is shown in movies of the next few pages.

Some of the energy of the light is required to pull the electrons away from the metal (in fact this is one way of measuring the ionization energy of a metal atom).

Unless the beam of light has at least this much energy, it will not be able to remove electrons from the metal surface at all. If the light has more than the minimum energy required, then the extra energy will be given to the ejected electrons as kinetic energy of motion.

Therefore, it was expected that the more energy a light beam contains, the faster the ejected electrons would move.