AAS Meeting #193 - Austin, Texas, January 1999
Session 34. Teaching Astronomy in Colleges and Universities
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1

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[34.01] Why Don't We See in the Infrared?

K. Brecher (Boston U.)

Solar radiation reaching the surface of the Earth includes a wide range of wavelengths and frequencies, with the peak of intensity including the infrared, visible and ultraviolet regions of the electromagnetic spectrum. In fact, the largest number of solar photons reaching our eyes arrives in the near infrared. Why, then, don't we see the world in the infrared? Many astronomy textbooks, both elementary and advanced, argue that the human eye has evolved to act as a kind of `solar Wien peak detector'. They note that if one uses the effective solar blackbody temperature of 5770 K, then, according to Wien's displacement law, the peak of the solar flux occurs at a wavelength of about 503 nm. Rods in the human eye peak in sensitivity at about 507 nm. Therefore, according to this line of reasoning, the human eye has evolved to avail itself of the Wien peak of the solar spectrum. (However, it should be noted that the peak sensitivity of the three different kinds of cones range from about 420 nm to about 560 nm). This astronomical argument is, at best, misleading. The solar blackbody spectrum, evaluated as a function of frequency, has a peak at about 340 THz. This is in the near infrared at a wavelength of about 884 nm. Since the human eye acts as an efficient quantum detector of photons, this is the appropriate solar radiation referent. Thus the Wien peak of the solar spectrum is not the reason that the eye's (scotopic) sensitivity peaks in the green. Many other factors contribute. Probably the most important are biochemical ones, which are constrained by available molecules such as rhodopsin that are useful for radiation to matter energy transfer. Physics also plays a role, because of absorption, scattering, refraction and diffraction. Other effects, including the Earth's atmospheric transmission and terrestrial surface reflectivities, may also matter.


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