The best way to look at it is the other way around.
Electromagnetic waves exist over a huge range of frequencies, from less
than 1 per second up to more than 3 billion billion billion per second.
If electromagnetic waves with frequencies between maybe about
4 million billion per second up to about 7.5 million billion per second
go into your eye, you SEE them.
Outside of that range, your eye doesn't know that there's anything there. But
IN that range, your eye has incredible nerve ends in the back that can tell the
difference between different frequencies. The highest frequencies in that range
produce the feeling of blue in your brain. The next highest ones make a feeling
of green. The next highest ones make the impression of yellow, and so on and
so on, down to the lowest frequencies that your eye can detect, and those look
red. If the frequency is any lower than red, you don't see it even when it pours
into your eyes.
