colourless

Colourless describes a property of a material, solution, or light that lacks perceptible colour in the visible spectrum. An object is colourless if it transmits or scatters light without selective absorption of particular wavelengths, so it appears transparent or nearly transparent under normal illumination.

In optics and physics, colour arises when a material absorbs some wavelengths and transmits or reflects others. Colourless substances absorb very little (or absorb uniformly) across the visible range, giving little or no hue. Impurities, thickness, and scattering can give faint tints or opalescence.

In chemistry, colourless solutions contain ions or molecules that do not produce colour by absorbing visible light. Water, sugar solutions, and many inorganic salts are colourless. By contrast, many transition metal ions (for example, copper(II), iron(III)) impart distinct colours. Some colourless substances can become coloured upon oxidation, reduction, or complex formation.

In gemology, colourless describes stones with no detectable colour; colour grading of diamonds ranges from colourless (D–F) toward faint yellow or brown (G–Z). Colourless glass and plastics are used for optical components and lenses.