Blackbody Radiation
Max Karl Ernst Ludwig Planck (1858–1947) is considered to be the founder of quantum theory, and therefore one of the most important physicists of the twentieth century. He was hired by an electric company to help invent an efficient lightbulb. In this task, he tried to figure out the relation between the intensity of light as a function of its wavelength from a hot, glowing filament. This is called the blackbody radiation problem. Current ideas about radiation from a hot body didn’t fit observations, especially at short wavelengths. He decided simply to find a mathematical expression that fit the observed intensity curve. Once he had a satisfactory fit, he was led to the conclusion that energy of the light was directly proportional to its frequency.
E = hν
This became known as a unit or a quantum of energy. Planck’s postulate was employed by another physicist to explain the development of current when light illuminated metals, the photoelectric effect.
The photoelectric effect
Albert Einstein (1879–1955). Einstein had a pretty good year in 1905. One of the papers he published that year was a simple explanation of the photoelectric effect. Einstein argued that the kinetic energy of photoelectrons was simple equal to the photon energy, as proposed by Planck, less the amount necessary to free the electrons from their valence band.
Thus was born the theory of quantum mechanics. Einstein’s work on the photoelectric effect was cited when he was awarded the Nobel Prize in physics.
In 1905, Albert Einstein also published a paper describing his special theory of relativity. He published another on the equivalence between mass and energy. He also published another paper analyzing Brownian motion and its relation to viscosity and diffusivity. Wow, that was a good year!