1 Atomic emission (line) spectrum

• When the atoms of a pure element in the gaseous or vapour state are heated to a high temperature, or are subjected to an electric discharge at low pressure in a discharge tube, they emit radiation.
• When this light is passed through a slit and a prism in a spectrometer, it does not give a continuous rainbow.
• Instead it appears as a limited number of coloured lines, each of definite wavelength and frequency, separated by dark regions.
• It is therefore called the line (emission) spectrum.

• Experimentally, the line spectrum of every element is a characteristic property: no two elements have the same line spectrum, like a fingerprint.
• For this reason spectra are used to identify elements (for example in stars and in flame tests).

The visible spectrum of hydrogen (discharge tube containing H₂ at low pressure) shows four lines: violet (410 nm), blue (434 nm), green (486 nm) and red (656 nm).