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Blackbody
Radiation
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Physlets page.
How to Use this Applet
The temperature of the black body applet can be set in two ways. The first
is to set the actual temperature by typing in a new value in the text field.
The applet will then show the spectrum and the apparent color of the object.
The second way is to move the peak wavelength by click-dragging on the graph
itself. The temperature is related to the peak wavelength according to
Wien's Law (T = 2.9E-3/lambda_max) -- which can lead to temperatures much in
excess of 10,000 Kelvin. Note the change of scale on the y axis. The total
energy radiated by a black body increases dramatically with temperature,
however, the scale is automatically changed to highlight the change in
color.
A simulation of the visible spectrum is displayed under the curve,
corresponding to 400 nm (blue), 500 nm (green), and 600 nm (red)
wavelengths. The colored circles on the left represent the percent of each
color present, and a simulation of the total color of the object.
What is a Black Body?
The color of an object depends upon its temperature as predicted by the
Stefan-Boltzmann law. This law describes the "ideal" blackbody radiation
given off by any object above absolute zero. Every object emits blackbody
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 visible light. For example, the heating
element on a stove glows "red hot". The Stefan-Boltzmann law allows us to
determine the temperature by measuring the color spectrum given off by the
heating element. Our Sun is an another example of a "real" blackbody
radiator. It's spectrum isn't as smooth as the "ideal" (it is pitted and
bumpy due to real-world conditions including, but not limited to, absorption
of the radiation en route to the earth) but it is close enough. It fact,
measuring color is the primary means astronomers have of determining the
temperatures of distant stars.
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