The Sun and the wonders of light
d ~ 223'833'600 km: day 87
The sun - that giant ball of boiling gravity-driven nuclear reaction - has been blasting us with electromagnetic radiation for billions of years. "Radiation" is a word loaded with bad connotations, but actually we live surrounded by it and depending on it every day. We can see and feel some of that radiation - yet our eyes and skin experience only a very small range of the sun's rays down here in the murky depths of our atmosphere.
Electromagnetic radiation travels from the sun as waves and there's a whole spectrum of wavelengths, going from nano-meters short to kilometers long. Each wavelength totally changes how we experience that radiation and what we call it. The smallest waves we get from the sun are x-rays, which are small and energetic enough to pass through your flesh but not quite through your bones. A little larger than x-rays are ultraviolet rays which we know from all the professional advice on sun-blocking creams can be damaging to your skin cells. Next up we have our good friend visible light, followed by infrared. Microwaves are the next biggest and finally we come to radio waves - a really broad category with wavelengths up to tens of kilometers long.
Why, bathed in all this variety, have our eyes developed to see only visible light? Just as objects around us are coloured because their molecules absorb or scatter certain wavelengths of light, different particles and molecules in the atmosphere also do the same. We don't see this because visible light lies in what is called an "atmospheric window" - it can travel relatively freely through the atmosphere without being absorbed or scattered along the way. Having said that visible light doesn't pass through all atmospheric objects easily; water droplets are particularly good at scattering visible light, which is why we can see clouds. But if our eyes were tuned to a wavelength which was not in an atmospheric window, we would just be surrounded by a permanent fog of light scattered by the atmosphere. And that would be pretty useless for getting around.
There are other atmospheric windows, though and you might wonder why we haven't chosen one of these? Light travels easily in the "middle infrared" for example - the wavelength of heat. If we could tune our eyes to see in this window then we would live in a dimmer world where warm things shine in their own light. Without temperature contrasts, objects of the same temperature would tend to blend into one and cold-blooded animals like snakes or crocodiles may well be perfectly camouflaged. There is also a third major atmospheric window at the wavelength of radio waves, which is why we use this for communicating. If our eyes saw through this window then we would see an alien world - lit up by a dim sun, radio transmitters and the odd pulsar.
We use the visible because here the sun is at its brightest and it is brighter than anything else on Earth or in the sky. This gives us the best chance of seeing our landscape clearly. We can walk around our world safely, avoiding objects as visible light is scattered off them. And this means that - among other things - we can devote a bit more time to enjoying the beauty of light and the things it allows us to see.