Chromolithographed solar spectrum from Bezold's 1876 Plate I: a band running from red at left through orange, yellow, green and blue to violet at right, crossed by thin dark lines lettered A to H above, beside a white square numbered 1.

Concept · Foundations

Light and the Spectrum

Visible light is the narrow band of electromagnetic waves the eye can detect, roughly 380 to 700 nanometres long.

Hue changes along the band, from red at the long end through orange, yellow, green and blue to violet. Sunlight carries the whole band and looks white; Newton Splits White Light shows a prism fanning it out. Dark lines, lettered A to H, mark light absorbed by gases in the sun and air.

A cut-glass crystal or a hose's spray in sunlight shows the band.
Magenta and purple appear nowhere in the band; the eye builds them from red and violet light together.
Beyond the two ends lie ultraviolet and infrared, waves too short or too long for the eye.

Nanometre (nm): One millionth of a millimetre, the unit used for the wavelength of light.

Image: Smithsonian Libraries via Internet Archive · Public domain