Revise GCSE Physics


Question:What are atomic absorption and emission spectra?

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The seven colours of the rainbow fit together to make the spectrum of visible light.

As we saw in section P1c, white light can be split (dispersed) into the seven colours of the rainbow by a prism.

We call the rainbow a 'continuous spectrum'; because there are no gaps beteen or amongst the colours.

Any light that is split into its component colours by a prism is called a spectrum, even if it was only some green light and some red light that were being separated.

Very hot substances (like the metal filament wire in a bulb) emit a continuous spectrum of visible light.

Most sources of light actually emit only a limited number of discrete colours.

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The above image shown, on the left, is of a type of lamp that emits light by passing electricity through hydrogen gas - called a Hydrogen discharge lamp.

The light at the bottom left of the picture is what actually comes out of the lamp.

When the light from the hydrogen discharge lamp is passed through a prism the spectrum on the bottom right of the same picture is produced as a result.

This spectrum is called the Emission Spectrum of Hydrogen. the number of lines and the colours of those lines are the same for every sample of hydrogen in the universe.

We say that this spectrum is characteristic of Hydrogen.

Every type of atom produced a unique emission spectrum.

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The spectrum of light emitted from an actual Hydogen gas source is shown below.

The diagram shows the different sorts of spectra that can be produced and the differing manners in which they are produced.

When a continuous spectrum of white light passes through a sample of cooler gas, we find the spectrum of the light that emerges contains missing colours which show up as black lines on the spectrum.

The missing lines are exactly the same colours that were emitted from the gas when hot, and the new spectrum is called the Absorption Spectrum of that material and is again characteristic of the material individual kind of atom used.

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This image shows the complete spectrum of light that is emitted by the Sun. The dark lines indicate that this is an absorption spectrum.

By analysing the position of the black lines we can work out which elements are in the Sun.