Concepts explained
The Eye Is a Very Narrow Window
It is easy to take the rainbow for the whole of light. It is a sliver. Remotes, microwave ovens, X-rays and radio are all light.
The empty place was the hottest. Which means something is there. Something we cannot see.
That is infrared. A thermometer was how humanity first learned that there is we cannot see.
The band the eye can take in is only this wide
The third piece said colour is, in the end, a size of ripple. Red long and unhurried, violet fine and tight.
Then there should be ripples longer than red and tighter than violet. There are. Our eyes simply do not reach them.
Keep going and it continues. Toward much longer ripples: the light inside a microwave oven, then broadcast radio. Toward much shorter: X-rays, then gamma rays.
radio, wi-fi, phones
remotes, thermal cameras, your own body
red to violet - a sliver of the whole
sunburn, sterilisation
the light that photographs bone
That all of these are one family was established in the mid-nineteenth century: light is a wave of electricity and magnetism pushing each other along, and the differences are of size, not of kind.
A little further in
This room is already full of light
Right now wi-fi is crossing your room, a remote control is waiting, broadcast radio is passing through the wall. All of it light. The room is packed with it.
And anything with a temperature emits light. Your body does. It is infrared, so you cannot see it. That is exactly how a thermal camera finds a person in the dark.
X-rays were found by accident too. Odd light leaked out during an experiment and lit a plate nearby; it passed through paper and wood alike. A hand held up showed its own bones.
The width of the window is no accident
Why this narrow band and no other? Not chance. It is the band the Sun pours out most of, and it is also the band that passes cleanly through air and water.
The eye was built around the most abundant light that actually arrives. We did not choose the size of the window; the environment cut it where it is.
So we believe we see the world, while looking through a very small gap. The history of telescopes and cameras is the history of widening it.
The question that remainsIf our band was set by our surroundings, would an eye grown under another star be seeing an altogether different world?