Colour of plasma

The colour of plasma is due to the energy from the relaxation of atoms, ions and molecules, released in the form of light. The wavelength depends on the gases present, each gas having its colour. Below are the colours of the gases most frequently used in plasmas. CF4: blue SF6: bluish white SiF4: light blue SiCl4: light blue Cl2: green white CCl4: green white H2: pink O2: pale yellow N2: orange red Br2: reddish He: violet red Ne: brick red Ar: dark red. Observation of the colour provides a qualitative idea of the purity of the gases present and any contaminations.