Light is the narrow band of electromagnetic radiation that is perceptible by the human eye. The visible rays produce a sensation of brightness and colour.

Visible light consists of vibrating packets or quanta of energy and differs from other electromagnetic radiation only in terms of wavelength. Light waves are considerably shorter, for example, than low-energy long-wave radio or radar waves. Wavelength is measured in nanometres (nm). One nanometre is a ten-millionth of a meter (10 ^-9 m).

Radiophysics

Two theories are used today to explain the radiophysics – the actions and principles – of light: the corpuscular theory of light developed by Isaac Newton, who proposed that units of energy are propagated at the speed of light in a straight line from the light source, and Christiaan Huygens' wave theory of light, which suggests that light moves in a similar way to sound. For more than a hundred years, scientists could not agree which theory was correct. Today, they are both used to explain the phenomenon of light.

Each spectral colour has a specific wavelength

In 1822, Augustin Fresnel succeeded in determining the wavelength of light. He proved that each spectral colour has a specific wavelength. His statement that "light brought to light creates darkness" sums up his realisation that light rays of the same wavelength cancel each other out when brought together in corresponding phase positions.

So each wavelength has a distinct colour appearance – and from short-wave violet through blue, blue-green, green, green-yellow and orange to long-wave red, the spectrum of sunlight exhibits a continuous sequence. The wavelengths of visible radiation range from 380 nm (violet) to 780 nm (red).