White light (e.g., sunlight) is actually composed of a rainbow of colours that the eye can perceive, as well as infrared and ultraviolet rays, which can not be seen. The colour of light is determined by its wavelength, measured in nanometers (nm), i.e., billionths of a meter.

The figure below represents the visible light spectrum, which varies in wavelength from 400 nm (violet) to 700 nm (red).


Different types of light bulbs emit different intensities of wavelengths. Light sources that appear white always emit a high proportion of blue light.

The figures below represent the light spectra of a mercury vapour bulb (on the left) and a high-pressure sodium bulb (on the right). Light emitted by the mercury vapour bulb appears very white and contains much more blue light than the sodium bulb, which appears more yellow. This means that the mercury vapour bulb produces much more light pollution than the high-pressure sodium bulb.

Light spectrum of a mercury vapour bulb.
Light spectrum of a high-pressure sodium bulb.
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