Expert: Philip Stahl - 2/21/2004

Question
Hello,
Does a thermal spectrum emitted by an object have a peak? or does it have the same intensity at all wavelengths?
What does the wavelength actually indicate?

I'm confused..

Thanks for the help,
-bob

Answer
Hello.

All radiating objects have a thermal spectrum that radiates at all wavelengths of the EM (electro-magnetic) spectrum, and have a particular peak.

The curve that discloses the peak is known as a 'black body curve'. The radiation is known as 'black body radiation'.

The point is that the position of the maximum ('peak') depends on the temperature of the object.

For example, the Sun has a BB curve that peaks at the 'yellow' region of the visible spectrum, though it radiates at all wavelengths. This position is set by the Sun's surface temperature.

If you go to a hotter star, say Sirius, you'll find the meximum of the BB curve has now shifted to a shorter wavelength position, since shorter wavelength conforms to higher frequency, higher energy radiation.

Thus, for any thermally radiating object there cannot be the same intensity at all wavelengths. Rather, Planck's radiation law shows how intensity of radiation varies with wavelength.

The peak of the BB curve for a given radiating object indicates where most of its radiation is coming from (e.g. 'yellow' in the case of the Sun), given a defined 'effective' (or surface) temperature.