The spectrum and the rainbow
White light holds every color. Newton proved it with two prisms, then picked seven colors to match music.
- Visible range
- About 380–700 nm
- Seven colors since
- Newton (orange and indigo added)
- Rainbow geometry
- Theodoric of Freiberg c. 1304–1310; Descartes 1637
Shine sunlight through a prism and it fans into a band from red to violet. Isaac Newton called that band a spectrum, Latin for an apparition. In his prism experiments of the 1660s he sent one color from a first prism through a second, and it stayed the same color. The prism wasn't adding color: white light is a mixture of all of them. It's the core of his Opticks.
The spectrum is continuous; nothing divides it into bands. Newton first named five main colors, then added orange and indigo so there would be seven, like the notes of a musical scale. That's why the rainbow is taught as seven colors, and why so many people can't find indigo in a real one. Physically, visible light runs from about 380 to 700 nanometers in wavelength.
A rainbow is the same spectrum painted on the sky by raindrops. Each drop bends sunlight, bounces it once inside, and bends it again on the way out, sending each color off at a slightly different angle, red on the outside of the bow and violet inside. Theodoric of Freiberg worked this out in the early 1300s using water-filled glass globes as giant raindrops, and Descartes completed the geometry in 1637. Long before, Aristotle had counted three colors in it.
Some colors you know aren't in the spectrum at all: magenta, pink, brown and grey. Your brain builds them; see colors that aren't in the rainbow. Bend the spectrum into a loop and close the gap with purples, and you have the color wheel.
Swatches
- Red · ~650 nm
- Orange · ~600 nm
- Yellow · ~580 nm
- Green · ~530 nm
- Blue · ~470 nm
- Indigo · ~445 nm
- Violet · ~410 nm
Colors
Sources
- Newton, I. (1672). A new theory about light and colors. Philosophical Transactions 6: 3075–3087.
- 'Music inspired Newton's rainbow'. Nature 520: 436 (2015).
- Boyer, C. B. (1959). The Rainbow: From Myth to Mathematics. Thomas Yoseloff.
Hex values are screen approximations. Every page lists its sources.