23.1. Blackbody Radiation and Planck’s Quantum Hypothesis http://www.ck12.org
We call an idealized body that absorbs all energy a black body because black does not reflect any of the electromag-
netic waves. Instead of reflecting the light that shines on it, a black body absorbs the light and its temperature goes
up. From the motion of the temperature, the electrons in it are vibrating and produce electromagnetic waves. Higher
temperature means faster vibration and higher frequencies.
The sun acts very similarly to a black body, because it releases its internal energy as light from the surface and does
not reflect incoming light. During the late 19th century, physicists were able to experimentally determine what
the shape of the curve for blackbody radiation looked like for objects at different temperatures, including the sun.
Figure23.3 shows two blackbody curves, one where the greatest intensity is at a temperature of 3000 K (a red star)
and the other where the greatest intensity is at 6000 K (a yellow star such as our sun).
FIGURE 23.3
The blackbody spectrum of two stars.The Classical Prediction
The problem with this observed result is that it did not match with the theoretical results from classic electromag-
netism. Classical electromagnetism treated the radiation as the smooth result of many different vibrations. With
many different vibrations within the body, the classical prediction was that shorter wavelengths were always more
common than longer wavelengths. There was no peak in the middle —only a peak against the extreme of wavelength
zero. This is known as Rayleigh-Jeans Law, after British physicist Lord Rayleigh (1842-1919) and Sir James Jean
(1877-1946).
The failure of this prediction was a puzzle for many years.