We are seeing rainbows everywhere at the moment, as families in lockdown brighten up their windows and neighbourhoods with multicolour creations. But have you have ever looked at an actual rainbow and wondered how they are formed?
The familiar rainbow we sometimes see in our skies is created when sunlight shines through droplets of water in the air. But a rainbow can occur whenever light shines through something that can bend the light and make it change direction; we call this ‘refraction’. The bending of white light splits it into colours; red, orange, yellow, green, blue, indigo and violet (often remembered as ROY G BIV or through the mnemonic ‘Richard Of York Gave Battle In Vain’).
The colours are actually different wavelengths of light; red having the longest wavelength and violet the shortest wavelength in what we call the ‘visible spectrum’.
Whist we are all aware of the colourful rainbow, what is less well known is that the spectrum extends further, with wavelengths of light that are invisible to our eyes. The rainbow is the visible part of the ‘electromagnetic spectrum’. The entire spectrum includes radiation that you may well have heard of, going from short wavelengths to long…
Gamma rays, x-rays, ultra-violet, visible light, infrared, microwaves and radio waves.
Throughout human history people have stared up at the night sky and seen the stars and planets, but objects in the Universe produce radiation over the whole range of the electromagnetic spectrum, which, until the 20th century, we were unable to detect.
The development of radio astronomy in the second half of the 20th century allowed astronomers to detect, for the first time, objects that are literally invisible; those that only give off light in the radio part of the spectrum. Soon after, technology began to be developed to allow detection of all wavelengths in the electromagnetic spectrum and today we have specialist telescopes for studying gamma rays, x-rays, infrared and ultraviolet radiation from objects in space. Some objects emit across a range of wavelengths, each revealing a different aspect of their composition and behaviour. Other objects are completely invisible at one wavelength, yet are clearly visible at another. Observing across the whole spectrum gives us a much better picture of our Universe. Astronomers today pursue ‘multiwavelength astronomy’ and may look at the same object in different wavelengths in order to understand it better.
This group of images taken of the Crab Nebula shows just how different the views at different wavelengths can be. The Crab is the remnant of a star that was observed to explode in 1054 AD, and recorded as the appearance of a sudden bright light in the sky. It is the result of a supernova explosion, when a massive star reaches the end of its life in one of the most energetic events in the Universe. At the centre of the nebula is a rapidly rotating neutron star which emits radio waves – also known as a pulsar.
The image in visible light reveals filaments of hydrogen gas still expanding into space away from the object’s core whilst the radio image reveals radio emission from electrons spiralling around inside the nebula. The nebula appears smaller and more compact in X-rays than other wavelengths because the electrons that are mainly responsible for the X-ray emission exist only near the central pulsar. For more information about the multiwavelength Crab Nebula check out this video from NASA.
Note on image colours: Please note that colours are used to highlight particular astronomical features and are not ‘real’ colours as would be viewed by an observer in space.