How does a sodium lamp work

An electrode is positioned at either end of the tube, and a large voltage applied. When current first passes between the electrodes, the neon is ionized to form a plasma, and starts to glow (as above), which explains why a sodium lamp first emits a pink shade before it glows with its characteristic orange colour. 

Atmospheric pressure is 1.01 x 105 Pa, so the pressure inside the tube is almost a vacuum. 

Sodium lamps glow pink before orange because of the neon they hold, which kick starts the sodium emission process. 

The lamp above is more properly called a low-pressure sodium lamp. Such lamps are ideal for street and road illumination, but the monochromatic nature of the emission makes seeing in colour impossible. An adaptation which emits a range of colours is the high-pressure sodium-vapour lamp, which is similar to that described above but contains a mixture of mercury and sodium. Such lamps emit a whiter light and are useful for extra-bright lighting in places such as road intersections, car parks and sports stadia. 

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