Photoelectric effect explained with visual content

The photoelectric effect refers to the phenomenon where light shining on a metal surface causes electrons to be ejected from that surface. Classical physics described light purely as a wave. It said the energy of light is spread smoothly over the wavefront, just like water waves deliver energy continuously to a shore. According to this view, shining a more intense (brighter) light should deliver more energy to the metal surface. It shouldn’t matter what color (frequency) the light is—just that the total energy is high enough. So classical theory predicted that if you used very intense light—even low-frequency red light—you should eventually eject electrons by pumping in enough energy over time.
But this is not what experiments showed. Experiments found that no electrons are ejected at all below a certain threshold frequency, no matter how intense the light is. Above that threshold frequency, electrons are ejected instantly—even at low intensities. Moreover, increasing the frequency of the light (making each photon more energetic) increases the kinetic energy of the ejected electrons, while increasing the intensity only increases the number of electrons emitted but not their energy. If the photon’s energy is greater than the work function, it gives the electron enough to break free. The extra energy left over becomes the electron’s kinetic energy. Quantum physics explains this by treating light not just as a wave but as photons, particles of light with energy E=h𝜗. Each electron absorbs the energy of a single photon. If the photon’s energy is below the metal’s work function (the minimum energy needed to release an electron), nothing happens—no electrons are ejected no matter how many photons arrive. If the photon’s energy exceeds the work function, the excess energy becomes the kinetic energy of the emitted electron. That explains why frequency determines whether electrons are ejected, while intensity controls only the number of electrons emitted (more photons means more electrons, but not more energy per electron).