Black Body
Black Body Radiation And Photoelectric Effect
Quantum Mechanics
started with Max Planck's hypothesis of discrete energy transitions in the absorption and emission of light by matter. With this hypothesis, he could explain the peculiar behavior of observed black body radiation. Classical thermodynamics predicts that for an ideal perfectly radiation-absorbing/emitting body (black body), the energy density of emitted radiation would exponentially increase with decreasing wavelength (ultraviolet catastrophe). In reality, one sees a temperature-dependent maximum beyond which the energy flux density goes down again. The figure here shows the black body radiation for different temperatures as derived from Planck's quantum hypothesis, in superb accordance with experimental observations.
Planck's hypothesis was later generalized by Albert Einstein
who postulated that light (electromagnetic radiation) itself is quantized in energy packets called photons. The experimentis crucis
for this idea was the photoelectric effect, the ability of light to deliver electrons out of certain materials. It was observed that this effect showed a sharp threshold behavior: beyond a certain wavelength, no free electrons can be generated whatever the light intensity is (left vacuum tube bombarded with long-wavelength "red" photons), whereas below some sharp wavelength threshold, photons can free electrons (right vacuum tube bombarded with short-wavelength "blue" photons). This can be easily explained when assuming a quantized-energy particle nature of light.