How light is made random motions of atoms and molecules.

We measure how hot something is by its temperature in Kelvins.

Inside matter the atoms (or molecules) are always moving. For instance, a gas (air, inside the Sun ...) might look something like this

The atoms fly about and bump into each other. The hotter the gas is, the faster the average speed of the atoms is.

As the atoms move and change directions, they emit electromagnetic radiation.

Sometimes you can see this radiation, somethimes you can't. The inside of a kiln used for melting glass to make beads and other small decorations is an approximate example.

There is an ideal case of this, called blackbody radiation. Many real cases of warm or hot bodies emitting radiation are pretty close to this ideal case.

Blackbody radiation is emitted over a range of wavelengths.

The amount of radiation and the distribution as a function of wavelength depends on the temperature (but not on the material in the ideal case of blackbody radiation)

Hotter --> more radiation.
Hotter --> smaller wavelenghts.

The quantitative relations are

The total energy in the radiation emitted by a hot body per unit time per unit area is proportional to the 4th power of the temperature (in Kelvins):
The wavelenght where the intensity peaks is inversely proportional to the temperature:

Thus if you double the temperature, the body emits 16 times as much energy per unit time, and the dominant wavelength is half as big.

We will use the T⁴ law later. For now, concentrate on how the wavelength varies with temperature. It gives us important information on the question, " How hot is the Sun?"

ASTR 122 course home page

Updated 1 October 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA