Expert: Courtney Seligman - 2/14/2011

Question
Hello Professor Seligman -

I picked up a NASA brochure on electromagnetic radiation at the recent AAS conference in Seattle. It described an experiment where William Herschel used thermometers to measure heat produced from various spectra of light. He found that heat increased as he went from blue to red; he then discovered infrared, which was warmer still. Here is my question: why does the heat increase with longer wavelength, when the shorter wavelength radiation has greater energy?

Thanks in advance.
Bill

Answer
Individual photons of light do have more energy, the shorter the wavelength of the light. But the total energy at a given wavelength also depends upon the number of photons. For instance, you could have two lights, one of which produces a hundred watts of red light, and the other a hundred watts of blue light. Since the blue photons have more energy per photon, fewer of them would be required to have the same total energy; but as long as there were enough red photons, the red light could have the same total energy, or more.

As for the increasing total amount of energy at longer wavelengths observed by Herschel, what that means is that there were more photons of long wavelength, so that despite the individual photons having less energy, the total energy was more than made up by the number of photons. I forget what his light source was, but it was presumably something which happened to produce most of its energy in the form of infrared radiation, and less (total) energy at shorter wavelengths. (As an example, today's incandescent light bulbs do just that, producing 95% of their energy in the infrared, which is why they are considered inefficient at producing visible light.) If he had used sunlight for the experiment, the peak of heat energy would have been in the middle of the visible spectrum, with gradually less and less heat as he moved toward the red, and then into the infrared. But any other source of light available to him would have, like incandescent bulbs, produced more heat at longer wavelengths.