Albert Einstien had a profound impact on our understanding of light.
Not only did he show that light is the universal
speed limit, but he put forth the idea of a quantum of light. A quantum is
just a small, indivisible bundles of energy. In essence, he was thinking of
light as a particle. (Note: Sir Isaac Newton had the idea of light as a
particle first, calling them "corpuscles". Most scientists ignored this
idea of his because, at the time, it didn't really help to understand light
better.) Light wasn't continuous, but came in little specks. The quantum of
light is called the photon. Photons can have different energies,
depending on their frequency. Higher frequencies of light are composed of
higher energy photons. This is expressed in equation:
Einstien also used the photon to explain the photoelectric effect. This was a curious phenomenon where light shining on a metal surface caused electrons to be flung off the surface. Once measured, it was discovered that these electrons had energies that would never exceed a certain maximum amount. Using a brighter light increased the number of electrons, but not the maximum energy found in any given electron. Increasing the frequency of the light did increase the maximum energy. However, the wave theory of light predicted that increasing the brightness (amplitude) of the light would increase the energy of the photons, since waves of higher amplitude have higher energy. Different frequencies of light shouldn't have any effect. Einstien explained the discrepancy by saying that the electrons absorbed an entire photon, so the could only gain as much energy as the photon had. Since the photon energy is only controlled by the frequency, the only way to increase the energy of the electrons is to increase the frequency (as the experiment showed). This particle view went on to be very important in quantum mechanics. The photon is the unit of all electromagetic radiation (see the EM spectrum).