Science strides through revolutions, but people
often refuse to accept revolutionary concepts at first place. This
describes the emergence of the Planck's equation. In classical physics,
energy of electromagnetic (EM) radiation was thought to be absorbed
or emitted continuously. It wasn't until late 1900 the German scientist
Max Planck (1858-1947) made a radical assumption in explaining the
black body radiation spectrum, the idea of discrete energy arose.
In Planck's assumption, radiant energy is emitted
in small bursts, known as "quanta". Each of the bursts called a
"quantum" has energy E that depends on the frequency f of
the electromagnetic radiation by the equation:
Photo of Max Planck.
Courtesy of AIP
Emilio Segre Visual Archives, W.F. Meggers
where h is a fundamental constant of nature, the "Planck constant".
is later found to be true for all EM radiant energy emitted or absorbed.
Planck's equation implies the higher the frequency of a radiation,
the more energetic are its quanta. It for example explains why you can never
get brown from visible light ((
but from ultraviolet light (from to ).
The quanta of visible light don't carry enough energy to start the chemical reaction
in your skin!
Figure: Visible Spectrum.
Courtesy of NASA.
The quantum energy is not to compare with the power of the light!
The Power of light (Luminosity) is the total energy per second, that means the number of quanta per second times the quantum energy.
Therefore even if visible light carrys a lot more Energy per second than UV-light, you won't get any browner from it.
The theoretical black body radiation spectra predicted
by Planck's Radiation Law, with the assumption , agreed
with the experimentally found spectra in all wavelengths and temperatures.
Photo of Max Planck with his hand writing.
Courtesy of the Archives, California Institute
Many other scientists, including Wien, Rayleigh and
Jean, attempted to explain the blackbody radiation spectra using classical
wave theory and failed. However, the idea of quantized energy was too revolutionary
for most scientists at the time (Even Planck puzzled his own conclusion).
It was not generally accepted until 1905 when Einstein extended
Planck's equation in deriving his formula for photoelectric emission.
The idea of quantized energy led Einstein to postulate the particle-wave
duality of light and other EM radiation. Planck's equation is
essential to the formulation of quantum physics.