In 1864, the British physicist James Clerk Maxwell proposed the mathematical theory of electromagnetism. According to his theory, the influence that changing electric fields and magnetic fields have on one another allows for the travel of waves. Maxwell's theoretical waves had the exact mathematical properties that had been measured for light. The vibrating electric charges that produce light are the electric charges in the atom. Maxwell's work gave the wave theory of light a solid foundation.
Maxwell's electromagnetic theory also did away with an idea that had stood in the way of scientists' acceptance of the wave theory for more than a century. Scientists felt they had to find the medium through which light waves travel. They reasoned that if light travels as waves, there must be something for them to travel through. But for light, this something could not be matter, because light can travel in a vacuum. To get around this difficulty, scientists suggested that the medium light travelled through was the ether.
All attempts to observe or measure the properties of the ether failed. Scientists became increasingly convinced that the ether did not exist. Experiments conducted by Albert Michelson and the American physicist Edward Morley in 1887 helped destroy the ether theory.
In 1900, the German physicist Max Planck discovered an equation that matched experimental data about the emission of light by a hot surface. Planck could not explain why the equation worked. But he realized that it predicted that the tiny emitters of light on the surface could have only certain values of energy. When energy is restricted to certain values, it is said to be quantized.
In 1905, Einstein revealed that light itself is quantized. Einstein reasoned that if light emitters can have only certain values of energy, then the energy they emit as light would retain its quantized character. The light comes in tiny packets of energy that are known as quanta. The concept of light as quantized energy explained how light behaves as a particle in certain experiments, instead of as a wave. These particles of light came to be called photons.
In 1913, the Danish physicist Niels Bohr proposed that the energy of atoms was also quantized. When energy is given to an atom, either by a collision or by shining light on it, the atom can accept only certain values of energy. In this way, the atom becomes excited. When it de-excites, it must get rid of the extra energy. One way it can do this is by emitting a photon that carries the energy away. Each type of atom accepts a different set of energies. Thus, when atoms emit light, the photons from one type of atom differ in energy from the photons from other types of atoms.
