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Why can't humans see in infra red? (Carissa D. FWMS)

 Hi Carissa,

That's a great question!  Unfortunately, I think it is one that lies outside the areas of expertise of the Solar Week scientists.  We know a lot about the Sun, the solar system, physics, and astronomy.  This is more of a biology question, but it is tied to the structure of the Sun and its spectrum.  The peak wavelength of the Sun's spectrum happens to lie within the range of light that humans can see.  We can assume that our eyes developed to make the most of the radiation emitted by the Sun that reaches the Earth's surface.  However, there are insects that can see at ultraviolet wavelengths.  The eyes of these insects developed differently than ours, because at some point in their evolution, it became advantageous to detect light at these wavelengths. 

Kris 

Great question! An important reason that humans, and most animals, do not see wavelengths that neighbor the visible ones, even though the Sun produces significant ultraviolet and infrared radiation, is that the ultraviolet and infrared rays do not penetrate below the upper few centimeters of the surface waters of the ocean.  Why should this matter to humans? Because we inherit our "vision equipment" from lower life forms that evolved in ocean waters. After those early single and multicell life forms evolved, later the evolution of the human brain modified human vision and other capabilities. Yet still human (and animal) eyes continue to "detect" best in the visible wavelengths, because that is all that was detectable by our evolutionary ancestors that did not live on land. [e.g. Segelstein, D. 1981. The complex refractive index of water. M.S. thesis. University of Missouri, Kansas City, MO.]