A diagram illustrating the emission of electrons from a metal plate, requiring energy gained from an incoming photon to be more than the work function of the material.

Emission of electrons from a metal plate requires energy gained from an incoming photon to be more than the work function of the material.

Incident light on metallic surfaces will cause the emission of electrons, a phenomenon known as the photoelectric effect. Several aspects of the photoelectric effect contradict classical theory. No electrons are emitted for light below some cutoff frequency. Additionally, while the maximum kinetic energy of emitted electrons is independent of light intensity, the maximum kinetic energy does increase with light frequency. Einstein postulated that light striking the material was quantized, imparting energy equal to hf. In order to cause the emission of an electron, photon energy must exceed the amount of energy, f, the work function, necessary to overcome the binding of the electron to the metallic surface. Low frequency light will not cause the emission of electrons no matter the intensity because individual light photons do not possess energy in excess of the work function.

One of the quintessential experiments of early modern physics, the photoelectric effect is a favorite for the MCAT. The photoelectric effect is a frequent topic in medical radiology. A really important thing to remember is that the kinetic energy of the ejected electron does not depend on the intensity of the light. In other words, the kinetic energy depends on the energy of the individual photons, not how much total light there is. A high intensity radio source would not produce the photoelectric effect at all because no individual photons would have sufficient energy, even though there was high energy overall, while low intensity x-rays would lead to the photoelectric effect.