Spectra

Emission Spectra

An Emission Spectrum is the range of colours given out by a light source.  To view spectra, the light must be dispersed in some way for the range of colours to be seen. This can be seen naturally (a rainbow) or artificially using a spectroscope / spectrometer. 

There are two main types of Emission Spectra:-

1. Continuous Spectra.

2. Line Spectra.

Continuous Spectra

If a beam of white light is passed through a prism or a grating a thermal continuous spectrum will be seen. 

For visible light 

Violet Light ~ 400nm (short Wavelength)

Red Light ~ 700nm (long Wavelength)

Line Spectra

When the source of light is from a specific element such as a Sodium vapour lamp, a continuous spectra is not observed. The observed spectra is a series of Lines, corresponding to specific frequencies of radiation being emitted. 

The below diagram shows the Line Spectra for several different elements : 

Absorption Spectra 

By observing first our own sun and then other stars we see not a line spectrum as expected, instead we observe a cross between an Continuous Spectrum and a Line Spectrum, which we call a Absorption Spectrum:-

The black bands in the above heliograph correspond exactly with where we would expect to find Line Emissions for individual elements:-