The human eye, or more specifically the retina,
has three types of "cones"or "light receptors"
that are sensitive to different frequencies of
electromagnetic radiation (light). As it
turns out, the cones are sensitive to wavelengths
(or frequencies) around red, green, and blue
light. Incoming light (at visible wavelengths)
will generally stimulate all three types of
receptors to different degrees, and the brain will
combine this information into color and intensity.
We cannot see UV light because the light
receptors in our eyes are not sufficiently
stimulated by UV light. Part of the "problem" is
that the cornea and your eye lens absorb much of
the UV light. In addition, I think because of
the lens shape, the UV light is not focused on the
retina (which would make things blurry), but you
might want to research this to double check me.
Also, I'm not too familiar with how sensitive
the photoreceptors in our eyes are to UV light (if
enough of it got through), so you might want to
investigate that as well. I think some insects
see in the UV (honeybees, for example), as well as
some birds andreptiles. There are probably others
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