Spectroscope

Do you know what light is? It is not just a white glow. Light is like a band in which every instrument plays a different colour.

A spectroscope splits light into its individual colours — much as a rainbow forms when sunlight passes through raindrops.

Light separated into its colours is called a spectrum, and it tells us a great deal. Every element — hydrogen or sodium, say — produces a unique coloured barcode by which we can recognise it. That is how we can tell what distant stars and planets are made of without travelling to them.

Look into the spectroscope and you will see the spectrum of sunlight. You may notice something odd: thin dark lines interrupting the rainbow. These are Fraunhofer lines, the coloured barcode of the chemical elements. The drop in brightness at a Fraunhofer line is caused by light being absorbed in the Sun’s gaseous envelope or in the Earth’s atmosphere.

Spectra come in two kinds, emission and absorption. An emission spectrum is produced by hot gas itself — in a discharge lamp, for instance — radiating energy; we then see several bright coloured lines on a black background. An absorption spectrum arises when continuous light (a full rainbow) passes through cooler gas, which absorbs photons at specific wavelengths matching its composition. The result is a coloured band interrupted by dark lines. This is the kind of spectrum we observe in the Sun, where cooler outer layers absorb part of the radiation from the hot core.