The visible spectrum is light perceived by the human eye, usually defined as wavelengths between 400-700nm. Some people can see as low as 380nm and as high as 780nm. Humans can perceive X-rays through second-order interactions. The spectrum closely matches short-wavelength light that penetrates the Earth’s atmosphere. Isaac Newton demonstrated the existence of the spectrum, which includes the colors of the rainbow. The human eye is most sensitive to green light, likely an adaptation to heavily vegetated environments. Brown, pink, and magenta are not true physical colors. The optic nerve and visual cortex are well-studied areas of the brain.
The visible portion of the spectrum is that which can be perceived by the human eye. It is slightly different for each person, although light with a wavelength of 400 to 700 nm is the usual definition. Some people are able to see visible light with wavelengths as low as 380nm and as low as 780nm. There is a repeatable experiment in which humans are able to perceive X-rays, which have wavelengths from 0.1 to 10 nm, but visibility can arise from second-order interactions that produce light in the visible range.
The portion of the spectrum in which visible light can be found corresponds closely to the short-wavelength light (less than 5 cm) which best penetrates the optical window of the Earth’s atmosphere. If humans evolved on a different planet with a different optical window, the range would likely closely match that light that passes through the atmosphere more easily.
The existence of a distinct spectrum was most famously demonstrated by Isaac Newton in his early experiments with prisms. He showed that white light is actually a composite of various types of light in the visual spectrum. These are also the colors that appear in the rainbow. A mnemonic for the visual spectrum is ROY G BIV: red, orange, yellow, green, blue, indigo and violet. The human eye is most sensitive to green light, with a wavelength of about 555 nm, likely an adaptation to help people navigate heavily vegetated environments, such as forests and jungles.
Brown, pink, and magenta are absent from the visible light spectrum, because they are not true physical colors, but instead emerge from certain combinations of light, particularly red. The optic nerve and visual cortex are among the best-studied areas of the human brain, giving us unique insight into how people process light.
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