Expert: Tom Whiting - 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
Bob,
I can't think of a single case where ANY spectrum does
not have a peak somewhere, which represents the most
intense wavelength of radiation....from a light bulb
to a star.
The wavelength represents that value which is the peak
of the radiation emitted, and you can think of it as
the perceived "color" of the object and/or the primary
temperature....as very hot things (say blue white stars) radiate (peak) mostly in the ultraviolet, whereas relatively cool things, like human bodies, radiate (peak) primarily in the infrared (heat)part of the spectrum.

Our sun peaks in the visible light portion of the electromagnetic spectrum, primary near the yellow color, thus we perceive our star as yellow color, represented by a surface temperature of around 11,000 degrees F.  (6000 degrees C.) or about 6300 degrees absolute (Kelvin).
Thus there is a direct relationship between peak spectrum
value, and temperature.
Hope all this helps,
Clear skies,
Tom Whiting
Erie, PA



FOLLOW UP:
Oh, perhaps your question was about wavelength....
well, emitted photons of light (or any other electromagnetic radiation for that matter) can be thought of as packets
of energy in wave form...and the wavelength is simply
the distance, measured in millionths of an inch, between
two peaks or two valleys in the sine wave of the photon...
The higher the temperature of the radiating body, the
shorter the wavelength...distance between (as explained
above) and the cooler the emitting body, the longer the
distance....so change in temperature only changes the
wavelength, not the speed as all light travels at c, the
speed of light.

(this wavelength value also is what changes when a body
is either coming toward you (blue shift) or is receding
from you (red shift), and not the speed...this is called
the Doppler Effect....but that's a different question.
Hope this helps,
Clear skies,
Tom Whiting