HISTORY OF SCIENCE AND TECHNOLOGY

ABSORPTION SPECTRA

When a source of white light, such as a lamp, is shone through a small slit in a piece of card, a narrow beam of light is produced. If a prism is placed in the path of this narrow beam of light then the light will be split into a range of colours – the colours of the rainbow, also known as a spectrum. This spectrum can be projected onto a white screen. The colours are red, orange, yellow, green, blue, and violet.

When a body is hot enough it gives out light. Which colours are given out depends on how hot the body is (its temperature) and what it is made of (what “elements” it contains). The sun is so hot that it gives out all the colours of the spectrum. It is a source of white light.

However, the sun is surrounded by an “atmosphere” (called the heliosphere) which is less hot than the sun itself. When the light from the sun passes through this cooler region, some of the colours of the white light are absorbed. When this light is passed through a prism we can see dark bands in the spectrum corresponding to the colours which have been absorbed. These dark bands tell us what elements are present in the gases surrounding the sun.

Every chemical element has its own spectrum. This spectrum is like a “fingerprint” for the element.

Each star will produce a different spectrum. Scientist can tell from the dark lines in the spectrum which elements are present in the star’s atmosphere. The Sun’s spectrum shows dark lines that represent absorbed light from the elements hydrogen and helium.

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