Why Can We Only See Visible Radiation?
In a way, the answer is simply a definition. Visible electromagnetic radiation, or light, is defined as the wavelengths that we can directly perceive with our visual systems. The greater question is why our visual systems evolved to respond to those particular wavelengths of electromagnetic radiation when there is such a vast range of wavelengths in the full spectrum. For example, why don't we see X-rays, or radio waves, or ultraviolet energy, or infrared wavelengths? Some insects actually do respond to ultraviolet energy, so clearly it is possible.
The true answer is that there are probably many reasons, functioning in combination, that resulted in our visual systems' functioning the way they do and responding to the range of wavelengths that they do. These include both physiological and ecological reasons. Physiologically, ultraviolet radiation (UV) is potentially very dangerous (and deadly) to biological tissue. Thus it would not serve us well to rely on detection of dangerous radiation to function in day-to-day living. Ultraviolet radiation is sometimes used to kill bacteria and other organisms. It would also damage our cells in a similar way. UV causes sunburn and also contributes to the development of cataracts (opacity) in the lenses of our eyes. Shorter wavelengths have even higher energy levels and can potentially cause more damage and/or pass right through us (like X-rays). So the UV end of the spectrum seems to be a reasonable limiting factor at the short-wavelength end of the visual spectrum. At the longer wavelengths, we have infrared radiation. It turns out that our body produces infrared energy simply because we are warm and that background radiation makes it difficult to detect infrared radiation from the environment (sensors in infrared cameras are cooled to very low temperatures for this reason). Thus visual noise might well be the limiting factor at the long-wavelength end of the spectrum. Even longer wavelengths, like radio-waves are so long that they pass right through (or perhaps more correctly around) us as well and we cannot detect them.
Ecologically, there are other reasons that narrow down the range of wavelengths we respond to visually. For one, the sun's peak energy output is very highly correlated with the wavelengths of light that we respond to. Thus we have a ready and plentiful source of energy to aid our visual perception. Additionally, many of the interesting interactions between electromagnetic energy and the elements and compounds we are made up of (as well as all the plants, animals, and objects we are interested in perceiving) happen in the visible wavelengths. Since the objects we are interested in perceiving modulate visible energy, and the cells we use to detect radiation are made of the same materials (and therefore are readily capable of detecting the light), it only makes logical sense that we would respond to these readily available and interesting wavelengths.
The bottom line is that we respond to the wavelengths we do because it is physiologically plausible for our visual systems to do so and because the information provided by such visual systems is tremendously useful to our survival.
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Ever wonder ... Why is a rainbow?
Updated: Apr. 19, 2011