We're all familiar with rainbows, those beautiful multi-coloured arches of light which span the sky during and after a rain shower. The reason those colours appear is because, as the sun’s light passes through rain drops suspended in the air, it's broken up into its component colours. The light from the sun isn’t just white light. The sun’s light is composed of every colour you can imagine plus many more colours that you can’t see because your eyes aren’t sensitive to them.

Finding out how stars work : Introduction to Spectroscopy

The great physicist and mathematician Isaac Newton investigated the nature of rainbows and their relation to the Sun’s light. In a simple experiment, he made a tiny hole in a blind, enough to let a narrow shaft of brilliant sunlight into his workshop. In its path, he placed a focussing lens and a glass prism. What’s a prism? It’s a piece of clear glass with three perfectly smooth surfaces.

Sunlight arriving at one surface is immediately broken down into its component colours. When the light leaves the prism on the other face, it is further broken down. You can then project the resulting rainbow, called a spectrum, on to a piece of card some distance away. This behaviour was well known in the scientific world so it wasn’t exactly news. However, Newton’s genius was to take the spectrum from the first prism and recombine it with a second prism, effectively reversing the action of the first prism. When the light from the spectrum was recombined, Newton observed an image of the Sun very similar to his original image.


Sunlight, passing through water droplets suspended in mid-air, is broken up into its component colours.	Newton's Experimentum Crucis was proof that white light was composed of coloured light rather that different-coloured light resulted from differing speed through a medium.

What’s the point of this experiment? Well, Newton was illustrating and expanding upon the well known property that light passing through a prism can be broken down into its component colours. This works for any light source. Recombine those colours and you are able to restore the original image. Newton was trying to figure out exactly what light was and how it behaved, and this experiment was significant to his emerging theory regarding the corpuscular theory of light.

Now, if you repeat the same experiment with a different type of light source, say a very bright candle then you will see a slightly different spectrum. The candle emits light as a result of burning candle wax, which is made of hydrocarbons and lots of impurities. Hence, the flame is a different colour to that of the sun. It has more yellow in it and less blue.

Physicists and chemists quickly discovered that burning different types of material in air resulted in different spectra although they weren’t exactly sure how to apply this knowledge.

Two centuries later, astronomer William Hyde Wollaston made a very important discovery. Expanding upon Newton’s work, Wollaston discovered that the spectra produced by passing sunlight through a prism was not uniformly distributed. He observed several dark patches up and down the spectrum, patches which remained in the same place from week to week. The German physicist Joseph Fraunhofer, examining Wollaston’s observations, noted that these dark lines could be very precisely imaged, noting that there were around six hundred such dark lines within the Sun’s spectrum.

Where do these lines come from?

The interior of the Sun converts raw hydrogen into helium by a process called nuclear fusion. It fuses two atoms of hydrogen together to make one atom of helium. At the same time, a small amount of energy is released as a photon of light.

This photon of light passes through the Sun’s outer atmosphere and, if the frequency of that photon matches the frequency of an atom within the Sun’s atmosphere then it is absorbed by that atom. If all of the photons of that particular frequency are absorbed then we see a corresponding dark line in the Sun’s spectrum. These lines are called Absorption Lines or usually Fraunhofer lines, after Joseph Fraunhofer.


Joseph von Fraunhofer (1787 - 1826)	The Sun's Spectrum - the black lines represent elements in the sun's upper atmosphere giving us an accurate measure of the composition of this region.

This was an incredibly significant discovery - it gives us a simple and effective technique for discovering the make up of the Sun’s atmosphere without having to go there in person. Better still, this technique works for any star. With a simple piece of equipment situated here on Earth, we can work out the composition of a star that is a million light years away. That’s powerful.

Wollaston’s and Fraunhofer’s equipment has been refined over the years into an instrument called a Spectroscope. Here, light from a distance object, is passed through a focussing element called the Collimator, and then into a prism. Once it exits the prism it is again brought into focus by an extra element, called the focussing lens. After that, the decomposed star light can be brought to a sharp focus on a screen, perhaps a photographic plate or, better still, an electronic detector.