Invisible light: the spectrum

You press the remote control and the TV obeys, although you see nothing flash. Your radio receives music straight through walls. And in summer you get sunburnt without visible light burning you. Behind all of this is the same thing as behind the light of a lamp: electromagnetic waves. Visible light is just one of many kinds, and most of them are invisible to your eyes.

From radio to gamma

Together, all these waves form the electromagnetic spectrum. It is sorted by wavelength, the distance from one wave crest to the next. At the long wavelength end are radio waves, with wavelengths from metres up to kilometres. Then come microwaves, then infrared, then visible light, then ultraviolet (UV), then X-rays, and at the short wavelength end gamma rays with wavelengths smaller than an atom. One simple rule holds: the shorter the wavelength, the more energy the radiation carries. Radio waves are harmless, gamma rays are dangerous.

Visible light: a tiny window

Your eyes only see wavelengths between roughly $400$ and $700$ nanometres, from violet to red. A nanometre is a millionth of a millimetre. Compared with the whole spectrum, that is a tiny slit, roughly as if you could hear only a single key of a huge piano keyboard. Everything else is still there: it is crossing the room around you right now, from the Wi-Fi signal to the heat radiation of your own body.

Light's invisible neighbours meet you all the time. Infrared sits right next to red, with a slightly longer wavelength: it is the heat radiation you feel on your skin from a radiator or a campfire, and thermal cameras make it visible. Your remote control also sends infrared flashes. Longer still are microwaves: in a microwave oven they make the water molecules in your food jiggle, and that jiggling is the heat that warms your meal. Ultraviolet sits on the other side, right next to violet, with a shorter wavelength: it is part of sunlight, causes sunburn and makes some materials glow under black light. Sunscreen is nothing but a UV filter for your skin. Shorter still are X-rays, which pass through soft tissue and show your bones at the doctor's.