To resolve the two puzzles in classical physics, Max Planck formed a simple
explanation, though by far the most important discovery in modern science.
After six years of "guess and check", Planck finally discovered an equation
that fit the experimental results of blackbodies quite accurately in 1900.
I(l,T) = 2pi * h * c^2 / l^5 * (e^(hc/lkT)-1)
where h is Planck's constant ( 6.6260755 * 10^-34 J*s). Contrary to the
classical
model Planck's formula predicted that as
the wavelength l approached zero, the intensity also went to zero,
instead of infinity.
In the explanation of his model, Planck made two bold and controversial
assumptions concerning the thermal radiation of matter:
1. The walls of the blackbody may emit radiation only at discrete units of
a certain energy En given by
En = n * h * f
where h is Planck's constant, f is frequency of molecular vibrations, and
n is a positive integer. Hence, the amount of energy emanated was required
to be a postive integral multiple of h * f. This was named the
Quantum Theory, since it allowed only quanta (plural of quantum)
of energies to exist. n is called the quantum number and the allowed
energy
states are called quantum states. This formula was the sole base
upon which the entire quantum mechanics was founded.
2. Molecules may absorb and emit only discrete units of light energy, as
described above. They do so by jumping from one quantum state to another.
The energy states differ by integral multiples of h * f. The quanta of
energy were termed photons. So molecules and atoms may receive
or give off photons to increase or decrease in energy, respectively.
A photon with frequency f has energy
E = h * f
So Planck's quantum theory explained the blackbody problem by introducing
the quantization of radiation into with photons.
The question of the photoelectric effect was explained in
1905 by Albert
Einstein by extending Planck's quantum theory to electromagnetic waves.
Einstein supposed that light (or any electromagnetic wave) was composed of
streams of photons, where each photon with frequency f had energy h * f.
When a light was shined on a photoelectric plate, the photons, as bundles of
energies,
donates all its energy to electrons in the metal. The electrons break free from
the atoms
and flow as current through the circuit.
Indeed, the quantum theory perfectly explained the mystery of the photoelectric
effect,
unsolvable by classical means.
