21.2 Introduction to Quantum Mechanics

The explanation for blackbody radiation and, in particular, the shape of the radiation curves, could not be found in classical physics.  For example, classical physics predicts that as an object heats up it should start releasing energy in shorter and shorter wavelengths, because this is the most efficient way to release energy.  At very high temperatures, the amount of energy released is maximized by the release of ever-shorter wavelengths.

Planck's formula represents a fundamental shift in theory.  In this system the amount of energy that can be exchanged between radiation and its surroundings is quantized, or limited to whole-number multiples of the smallest unit of energy-the quantum.  Furthermore, the value of each quantum is equal to the product of Planck's constant and the frequency of the radiation ( E = hf ).  The concept of the quantum effectively ended classical physics and gave birth to quantum physics.

Several years later Albert Einstein proposed that radiation, including visible light, could be quantized regardless of whether or not it exchanges energy with its surroundings.  In 1926 chemist Gilbert Lewis introduced the term photon  to describe a single quantum of light.  The energy of one photon is given from Planck's formula and is  E  =  hf .