Exploring Quantum Physics through Hands-on Projects

CHAPTER 2

LIGHT AS PARTICLES

Next time you turn on an electric oven, pay attention to the way in which the color of its glow changes as it heats up. The glow is at first weak and dull red before it turns bright orange. At higher temperatures, such as those in a blacksmith’s metalworking oven, the glow becomes much brighter and whiter.

All materials emit electromagnetic radiation with a very similarly colored glow as a function of temperature. To simplify the study of this phenomenon, physicists make use of an idealized object known as a blackbody. This theoretical device is able to absorb all electromagnetic radiation that reaches it, and then re-emits this radiation in a characteristic, continuous spectrum. Because this object reflects no electro-magnetic radiation (including visible light) it theoretically appears black when it is cold, and this is where it gets its name. However, at any temperature above absolute zero, the blackbody emits a temperature-dependent radiation known as blackbody radiation.

In 1905, British Nobel laureate Lord Rayleigh (his real name was John William Strutt) and his colleague Sir James Jeans derived an expression for blackbody radiation. They assumed that blackbodies are made of a huge number of harmonic oscillators—imaginary contraptions somewhat like guitar strings that can vibrate in response to a sound and can themselves produce a sound when they vibrate.

As shown in Figure 20a, a harmonic oscillator—such as an ideal elastic string held between ...