Physical entities such as light and electrons possess both wavelike and particle-like characteristics. This is known as wave/particle duality. On the basis of experimental evidence, Einstein first showed (1905) that light, which had been considered a form of electromagnetic waves, must also be thought of as particle, or localized in packets of discrete energy. Nearly twenty years later, the French physicist Broglie proposed that electrons and other discrete bits of matter, which until then had been conceived only as material particles, also have wave properties such as wavelength and frequency. Soon after, the wave nature of electrons was experimentally established. An understanding of the complementary relation between the wave aspects and the particle aspects of the same phenomenon was announced in 1928.
This dualism to the nature of light is best demonstrated by the photoelectric effect, where even a weak ultra-violet light produces a current flow - i.e., releases electrons but a red light does not release electrons no matter how intense the red light.
Physical entities such as light and electrons possess both wavelike and particle-like characteristics. This is known as wave/particle duality. If a beam of light is pointed at the negative end of a pair of charged plates, a current flow is measured. A current is simply a flow of electrons in a metal, such as a wire. Thus, the beam of light must be liberating electrons from one metal plate, which are attracted to the other plate by electrostatic forces. This results in a current flow. Normally, one would expect the current flow to be proportional to the strength of the beam of light - i.e., more light = more electrons liberated = more current. But it is observed that the current flow is constant with the intensity of the light, but varies strongly with a change in the wavelength of light such that there was a sharp cutoff and no current flow for long wavelengths.
In his scientific paper, Einstein showed that light was made of packets of energy quantum called photons. Each photon carried a specific energy related to its wavelength, such that photons of short wavelength (blue light) carry more energy than long wavelength (red light). To release an electron from a metal plate required a minimal energy i.e. the wavelength of the light had to be a sufficiently short. Each photon of blue light released an electron. But photons of red light were too weak. The result is no matter how much red light was shown on the metal plate, there was no current flow.
The photoelectric effect earned Einstein the Nobel Prize, and introduced the term "photon'' into our terminology.
