At the
end of the 19th century, no law had been discovered to account for
the phenomenon of heat and light radiation by a solid, white-hot body. In 1900
Max Planck (Germany, 1858-1947) guessed that radiation did not occur in a continuous fashion but in small discrete units, separate
quantities or quanta. This discovery, which enabled scientists to explain heat
radiation, turned physics upside down, especially in the sphere of classical
mechanics, which became inoperable in the area of infinitely small quantities. Thanks to this
theory, Albert Einstein (Switzerland, b. Germany; 1879-1955) explained in 1905
the photoelectric effect by showing that light, which comprises both waves and
particles, moves by quanta, tiny packets of light, which were later called
photons.
Niels
Bohr (Denmark, 1885-1962) built on this quantum theory
a model of an atom, describing in 1911 the movement of electrons inside the atom.
This model enabled him to achieve remarkable results in the fields of the
spectroscopy of gaseous matter and of X-ray physics. (‘Inventions and
Discoveries’)
