The whole spectrum: light we cannot see

A rainbow shows every colour your eyes can see, from red on one side to violet on the other.

But the rainbow does not really stop at its edges. It carries on in both directions, with kinds of light that eyes cannot pick up.

Beyond the red end is infrared. You cannot see it, but you can feel it: it is the warmth on your face from a fire. Further on are the radio waves that carry music to a radio.

Beyond the violet end is ultraviolet, which gives you sunburn. Further still are X-rays, which doctors use to see bones.

These are all light. Stars and galaxies shine in every kind. Astronomers build special telescopes for each one, and each shows them things the others cannot.

One thing, many sizes. Light is a wave, a ripple in electric and magnetic fields, and it travels through empty space at 299,792 km a second. What differs from one kind of light to another is the wavelength, the distance from one crest to the next. Shorter waves carry more energy.

Kind	Wavelength, roughly	A crest-to-crest distance about the size of	What it shows astronomers
Radio	Longer than 1 mm	A building, down to a grain of rice	Cold gas between the stars, the spinning cores of dead stars
Infrared	0.0007 mm to 1 mm	A pinhead, down to a cell	Stars hidden inside dust, cool things such as planets, the most distant galaxies
Visible	0.0004 to 0.0007 mm	A bacterium	Ordinary stars
Ultraviolet	Down to a hundredth of visible	A virus	Hot young stars
X-rays	Smaller again by a thousand	An atom	Gas at millions of °C, and matter falling towards black holes
Gamma rays	The shortest of all	The nucleus of an atom	The most violent explosions known

The part of the radio range with the shortest waves is often given its own name, microwaves. A microwave oven and a mobile phone both use it.

How they were found. In 1800 William Herschel spread sunlight into its colours with a prism and laid thermometers across them. The one he had put just beyond the red, where there seemed to be no light at all, warmed up the most. He had found infrared. Ultraviolet was found the next year. In the 1860s James Clerk Maxwell worked out that light is an electric and magnetic wave and that waves of other lengths ought to exist. Radio waves were made in a laboratory in the 1880s and X-rays were discovered in 1895.

Hot things glow. Anything warm gives out light, and the hotter it is the shorter the waves it mostly gives out. Your body, at 37 °C, glows in infrared. The Sun, at about 5,500 °C at its surface, shines mostly in visible light, and it is no accident that our eyes work in exactly that range. Gas at a million °C glows in X-rays. So choosing a kind of light is a way of choosing what temperature of thing to look for.

Why telescopes go to space. The Earth’s air blocks most of the spectrum. That is good for us: it stops the X-rays and most of the ultraviolet that would harm living things. Only visible light, a little of the infrared and much of the radio range get through to the ground. For everything else a telescope has to be above the air.

To see in	You need
Radio	A large dish on the ground
Visible	A telescope on the ground, or in space for the sharpest view
Infrared	A high, dry mountain for some of it; space for the rest, such as the Webb telescope
Ultraviolet, X-rays, gamma rays	A telescope in space, such as the Chandra X-ray Observatory

Putting the pictures together. A single object can look utterly different in each kind of light. A cloud that is black and empty in visible light may blaze in infrared with newborn stars. A galaxy that looks quiet may have jets a million light years long in radio. Most of what has been learned about the universe since the 1950s came from opening these new windows one by one.