Some objects emit and absorb radiation better than others. A blackbody is an object that absorbs all the electromagnetic energy that falls on the object, no matter what the wavelength of the radiation. A perfect blackbody does not exist, but it is a useful reference for determining how good a body is at emitting and absorbing radiation. While an object may visually appear black, it does not mean it is a blackbody. Using the applet below you can draw energy curves. The x-axis is wavelength in microns of the emitted radiation. The y-axis is the amount of radiant energy emitted at a given wavelength. The resulting energy curves assume that the objects emitting radiation are perfect blackbodies.

As you adjust the temperature of the emitting object using the slider at the top of the figure, notice:

• How the area under the curve increases as the temperature is increased,
• How the peak in the emitted energy changes wavelength,
• For a perfect blackbody, energy is emitted at all wavelengths, the issue is how much energy is emitted.

Can you use this tool to explain why the distribution of emitted radiation with wavelength varies so differently between the Earth and the Sun?

Area under the curve