What is light?

Credit: ESO

Our eyes can only see the small part of the electromagnetic spectrum that we call visible light, but radio waves, infrared and ultraviolet radiation, X-rays and gamma rays are also part of the spectrum.

Colours of the rainbow: A rainbow reveals that white sunlight consists of various colours. Each colour has a different wavelength.
Credit: A. Caproni (ESO)/ALMA (ESO/NAOJ/NRAO)
Windows on the Universe: Earth’s atmosphere blocks many types of electromagnetic radiation from the cosmos. From the ground, we can only observe the Universe through some “windows”. The atmosphere protects life on Earth from hard radiation from space.
Credit: ESA/Hubble (F. Granato)

The light that our eyes can see is only a small part of the so-called electromagnetic spectrum. To study other types of radiation, like radio waves, millimetre waves, infrared radiation, ultraviolet radiation, X-rays and gamma rays, astronomers need special instruments. Because of atmospheric absorption, some of these observations can only be carried out from space. In general, cold objects emit low-energy, long-wavelength radiation, while the hottest, most energetic events in the Universe produce X-rays and gamma rays.

Broadband vision: X-ray, UV, optical and radio observations of the active galaxy 3C321 are combined here into a single image.
Credit: NASA/CXC/CfA/D.Evans et al./ESA/STScI
A different kind of light
Spectral image: The electromagnetic spectrum extends from long-wavelength, low- energy radio waves all the way up to the highest energy gamma rays. Visible light is only a small part.
Credit: NSF/VLA/CfA/D.Evans et al./ STFC/JBO/MERLIN

Have you ever heard of invisible light? It’s light your eyes cannot see, like infrared radiation, which we feel as heat, or ultraviolet light, which causes sunburn. To learn about the Universe, astronomers study all kinds of light.