Planck's Constant

Planck's Constant, fundamental physical constant, symbol h. It was first discovered (1900) by the German physicist Max Planck. Until that time, all forms of electromagnetic radiation, including light, had been thought to behave only as waves. Planck noticed certain deviations from the wave theory on the part of radiations emitted by so-called blackbodies, which are perfect absorbers and emitters of radiation. He came to the conclusion that electromagnetic radiation is emitted in discrete units of energy, called quanta. This conclusion was the first enunciation of the quantum theory. According to Planck, the energy of a quantum of electromagnetic radiation is equal to the frequency of the radiation multiplied by a constant. In mathematical terms, this can be expressed:

E = h f,

where E is the energy of a single quantum, h is Planck's constant, the value of which is presently accepted as 6.626 × 10^-34 joule-seconds, and f is the frequency of the radiation. Using this equation, the energies associated with quanta of different frequencies of electromagnetic radiation can be calculated, for example:

• Typical energy of quantum of X-rays = h.f ~ 6.6 × 10^-34 × 10¹⁹ = 6.6 × 10^-15 J
• Typical energy of quantum of ultraviolet = h.f ~ 6.6 × 10^-34 × 10¹⁵ = 6.6 × 10^-19 J
• Typical energy of quantum of infrared = h.f ~ 6.6 × 10^-34 × 10¹³ = 6.6 × 10^-21 J

Planck's original theory has since had abundant experimental verification, and the growth of the quantum theory has brought about a fundamental change in the physicist's concept of light and matter, both of which are now thought to combine the properties of waves and particles. Thus, Planck's constant has become as important to the investigation of particles of matter as to quanta of light, now called photons. The first successful measurement (1916) of Planck's constant was made by the American physicist Robert Millikan.