Have you ever wondered how would the rainbow look like as
seen by an animal that has different vision spectrum than human? Just posing
this simple question should lead to a quick conclusion that we see just a part
of the rainbow – the part that shines with the colors we can perceive. The
rainbow extends both into infrared and into ultraviolet and the only reason we
can’t see it, is because our eyes can’t perceive these “colors.”
If you were able to see all parts of the electromagnetic
spectrum, you would see that the rainbow is made of a number of separated arcs of different width and brightness.
This is because the Earth’s atmosphere is opaque to many light frequencies, for
example high energy ultraviolet and most of the infrared spectrum. Also, keep
in mind that the reason why rainbow emerges in the first place is because light
rays going through water are scattered. Anything that cannot be scattered by
water droplets is then out of the picture too. This includes some microwaves (water
is opaque to microwaves, this is why microwaves boil water, duh!) and radio
waves (anything with wavelength comparable to or exceeding the size of a
droplet cannot be scattered by it).
As a matter of fact, if the lens in your eye was not opaque
to ultraviolet light, you would be able to see it (and some people do, after removal of their lenses)! An interesting fact is that ultraviolet is not
perceived by cones (which are the cells responsible for perceiving color) but lower wavelength ultraviolet is perceived by rods (which are the cells which perceive light intensity). Thus, if not for the lens, something
glowing in ultraviolet would just look brighter, but would not give it any
distinct color you could name. Your brain would not be able to
distinguish between something glowing green and ultraviolet and something
brighter glowing just green (or glowing green and white). If you looked at
the rainbow, the arc next to violet would just make the colors of the
background brighter.
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