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 short as 380 nm and as long as 780 nm. There is a repeatable experiment in which humans are able to perceive x-rays, which have wavelengths as short as 0.1 to 10 nm, but the visibility may derive from second-order interactions that produce light in the visible range.
The portion of the spectrum where visible light can be found corresponds closely to the short-wavelength (less than 5 cm) light that best penetrates the Earth atmosphere's optical window. If humans evolved on a different planet with a different optical window, the range would likely correspond closely to that light which passes through the atmosphere easiest.
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, probably an adaption to help people navigate in environments rich with greenery, 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, especially red. The optic nerve and visual cortex are among the best studied areas in the human brain, giving us unique insight into how people process light.