The electromagnetic radiation spectrum covers a wide range of wavelengths, from several kilometres long radio frequency waves to gamma rays with wavelengths the size of atomic nuclei, below 10-14m. Wedged between infrared and ultraviolet radiation is the visible spectrum, visible light or simply light, a very narrow range with wavelengths smaller than a thousandth of a millimetre, to which our retina is sensitive.
The different perceptions light produces in the eye are what we call colours, so the visible spectrum is in turn divided into bands, each of which we perceive as a colour: violet, blue, green, yellow, orange, and red. Vision is the result of signals transmitted to the brain by light-sensitive photoreceptor cells in the retina. There are two types of receptor cells, cones and rods, so called because of their external shape.
Cones are activated only by intense light, such as daylight, and are sensitive to wavelength or colour. There are three types of cones sensitive to long, medium and short wavelengths of the visible spectrum, coinciding with the red, green, and blue zones, respectively. This is why we are able to perceive a very wide range of colours. The cones concentrate mainly in the central area of the retina, where colour perception takes place. These cones’ vision is called day vision or photopic vision.
Conversely, rods can operate in very dim lighting conditions, such as on a moonlit night, but unlike cones, they are not sensitive to colour. Rods are abundant in the periphery of the retina (peripheral visual perception) where black and white is perceived. Rod vision is known as night vision or scotopic vision. That is why, when there is no light, we are not able to distinguish colours and everything looks the same in the dark.
Answered by Augusto Beléndez Vázquez, Professor of Applied Physics at the University of Alacant.
Question submitted by Pablo Rosillo Rodes.
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