The temperature of the BlackBody applet can be set in 4 ways:
A simulation of the visible spectrum is displayed under the curve, corresponding to 400 nm (blue), 500 nm (green), and 650 nm (red) values. The colored circles on the left represent the percent of each color present and a simulation of the total color of the object.
The color of an object depends upon its temperature as predicted by Planck's formula. This law describes the "ideal" blackbody radiation given off by any black (i.e. perfectly absorbing) object above absolute zero. Nonblack objects emit according to the "ideal" blackbody radiation spectrum multiplied (frequency by frequency) by the proper absoption coefficient characteristic of its surface.
Every object emits thermal radiation although you do not normally notice it because our eyes are only sensitive to a very small portion of the electromagnetic spectrum. An object must be quite hot for it to emit a significant amount of visible light. For example, the heating element on a stove glows "red hot". The Stefan-Boltzmann allows us to determine the temperature by measuring the color spectrum given off by the heating element. Our Sun is an a fair approximation of a "real" blackbody radiator. Its spectrum isn't as smooth as the "ideal" (it is pitted and bumpy due to real-world conditions including absorption of the radiation en route to the earth and non perfectly "black" surface) but it is close enough. In fact, measuring color is the primary means astronomers have of determining the temperatures of distant stars.
However, note how our eyes have a hard time to tell apart the white shades between 4500 and 6800 K! Same story above 50000 K.
More details on the theory behind this spectrum.
A similar applet, also illustrating the Stefan-Boltzmann law, is worth taking a look.
|created: 13 Sep 2001||last modified: 21 Oct 2004 by Nicola Manini|