“Es ist mir ein prächtiges Licht über die Absorption und Emission der Strahlung aufgegangen ‒ es wird Dich interessieren. Eine verblüffend einfache Ableitung der Planck’schen Formel, ich möchte sagen die Ableitung. Alles ganz quantisch.”
“A splendid light has dawned on me about the absorption and emission of radiation ‒ it will be of interest to you. A stunningly simple derivation of Planck's formula, I might say the derivation. All completely quantical.”
Albert Einstein in a letter to his friend Michele Besso on August 11, 1916.
The “splendid light” refers to Einstein's insight that stimulated emission (also called induced emission) of light from excited atoms occurs in nature, and that this yields an elementary explanation of Planck's formula for the spectrum of black body radiation.
And, of course, some 46 years later and 50 years ago this May, the “splendid light” of Einstein's idea became a real “splendid light” with the construction of the Laser, based on the principle of stimulated emission of radiation.