Modern Physics: Relativity and the Photon

By the late 1800s, classical physics explained nearly everything observed -- except light's behavior, which led Einstein to two revolutionary 1905 ideas. Special relativity rests on two postulates: the laws of physics are identical in every non-accelerating reference frame, and the speed of light in a vacuum (c = 3.00x10^8 m/s) is the same for every observer, regardless of how fast the source or the observer is moving. This second postulate has a startling consequence, time dilation: a clock moving relative to an observer runs slower by a factor gamma = 1/sqrt(1-(v/c)^2) as that observer measures it -- far too small to notice at everyday speeds, but real and precisely measured at high speeds, and even something GPS satellites must correct for to keep their timing (and therefore your location) accurate.

Einstein's second 1905 breakthrough treated light as also behaving like a stream of individual particles called photons, each carrying a discrete energy E = h*f = h*c/lambda, where h = 6.63x10^-34 J*s (Planck's constant) -- so tiny that photon energies are usually expressed in electron-volts (eV) instead of joules, where 1 eV = 1.6x10^-19 J. This explained the photoelectric effect: light shining on certain metals ejects electrons only if each individual photon carries enough energy to overcome the metal's work function (phi, the minimum energy needed to free an electron); a very bright light at too low a frequency ejects nothing at all, while even a dim light at high enough frequency ejects electrons instantly -- a result impossible to explain if light behaved only as a wave.