Emission spectrum

Also called Line spectrum

An emission spectrum is the set of bright lines an excited gas gives off, one line for each downward transition between two of its energy levels. The pattern of frequencies is unique to the element, so it identifies the source.

When an atom drops from a higher energy level to a lower one it emits a photon carrying exactly the difference, and 15.3.A.3 makes that a single frequency and therefore a single wavelength. Collect every transition a gas can make and the result is a set of bright lines against darkness.

Because each element has its own set of allowed levels it has its own set of frequencies, and 15.3.A.4.i gives the use directly: an emission spectrum can be used to determine the elements in a source of light.

The arithmetic is one substitution. A transition of size ΔE\Delta E emits at f=ΔE/hf = \Delta E / h, and λ=c/f\lambda = c/f converts. With ΔE\Delta E in electron volts the printed hc=1240 eV⋅nmhc = 1240 \, \text{eV} \cdot \text{nm} collapses both steps into λ=1240/ΔE\lambda = 1240 / \Delta E nanometers.

Keep this apart from blackbody radiation. A blackbody emits a continuous spectrum fixed by temperature alone; an excited gas emits a line spectrum fixed by its energy levels. One tells you how hot a source is, the other tells you what it is made of.

The CED's sample activity for Topic 15.3 has students view a hydrogen discharge tube through a diffraction grating and match the red, cyan and purple lines to the 3 to 2, 4 to 2 and 5 to 2 transitions.

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