Blackbody Radiation

Rutgers University Institute of Marine and Coastal Sciences

A blackbody is defined as an object that absorbes all incoming radiation and emmits all energy at full efficiency for all wavelengths. The amount of energy emmitted by a blackbody can be calculated for different wavelengths. The following equation is used to calculate the magnitude of this wavelength dependent radiation.

A plot of this equation for different temperatures is shown below. The x-axis is wavelength, and the y axis is radiation intensity (B).

As the temperature increases the magnitude of the peak increases and shifts toward shorter wavelengths. The following equation describes the temperature dependence of the curves:

Using the above equation the wavelength that yeilds the most radiation can be calculated for a given temperature. While the above equation is based on an ideal blackbody, approximations can be made for other objects. Wein's Displacement Law provides scientists with a means to calculate these peak wavelengths for any known temperature. The following figure illustrates the relative locations of the blackbody peaks for the sun and the earth.

**Note:The magnitude of the Earth curve has been magnified 500,000 times.

The radiation maximum for the sun and earth occur at two distinct wavelengths of radiation. The solar peak is located in the visible portion of the spectrum. Therefore it is probably not a coincidence that our eyes are very sensitive to this specific band of radiation. Other radiometers specific to this band can be used to measure reflected sunlight and detect cloud formations. It is this type od sensor that collects teh information that generates weather maps. The temperature of the earth is much less than that of the sun and consequently the radiation peak is smaller and pushed to longer wavelengths. The earth radiation peak corresponds to the infrared portion of the spectrum. Oceanographers have therefore designed radiometers that are sensitive to both the visible solar radiation peak and the infrared peak of terrestrial radiation.

Useful Blackbody Radiation Links

Blackbody Definitions

-No pictures, but a really good set of definitions

Pictures, Pictures, and more Pictures

-This page has an abundance of pictures to describe the properties of a blackbody

The Electromagnetic Spectrum