What if light had only one color?

After the rain, the Sun comes out. Look! A rainbow!

The rainbow's colors were hiding in the sunlight all along. What if light had only one color?

A rainbow seems to appear out of nowhere: red, orange, yellow, green, blue and violet stretched across the sky. Where do those colors come from? They were inside the sunlight all along. White light is a mixture of all the colors, and raindrops spread them out.

So what if light had only one color instead of many? This lab is a thought experiment. We follow the science step by step to imagine what might change.

OpenStax describes white light as a fairly uniform mixture of all visible wavelengths. The same process that breaks sunlight into a rainbow in raindrops breaks white light into colors in a glass prism or a diamond: dispersion, the spreading of light according to wavelength.

This thought experiment asks what would follow if all light had one wavelength: what would happen to rainbows, to the colors of everyday objects, and to a prism.

Sunlight looks white. But it has red, orange, yellow, green, blue and violet inside.

A raindrop or a glass prism spreads the colors out, so we can see them.

NASA explains that our eyes can see light with wavelengths from about 380 nanometers, violet, to about 700 nanometers, red. Each color is a different wavelength, and white sunlight contains all of them.

When sunlight passes through a glass prism, each color bends by a slightly different amount, so they fan out into a rainbow. NASA notes that Isaac Newton showed this with a prism in 1665. Raindrops do the same in the sky.

NASA's visible-light page gives the eye's typical range as 380 to 700 nm, violet shortest and red longest, and notes Newton's 1665 prism experiment showing that each color refracts at a slightly different angle. OpenStax's Table 1.2 shows why: water's index of refraction is 1.331 for red (660 nm) and 1.342 for violet (410 nm), so violet bends more.

In a raindrop, light refracts on entering, reflects from the back of the drop and refracts again leaving, which is why rainbows appear only when you look away from the Sun.

Now imagine all light is just one color, like only yellow.

There might be no rainbows. A red apple and a blue cup might look dark. The world might look like one color in light and dark shades.

Imagine all light was a single color. A raindrop could still bend it, but there would be nothing to spread out, so instead of a rainbow you might see just a band of that one color, or nothing special at all.

Objects would look strange too. An object's color comes from which colors of light it bounces back. If only one color of light existed, things that bounce that color would look bright, and things that soak it up would look dark. The world might look like a one-color photograph. These are ideas.

With a single wavelength, refraction would still occur, but dispersion would have nothing to separate: a raindrop or prism would redirect the light without producing a spectrum. Rainbows as color bands might vanish.

Object colors would collapse too. OpenStax notes that an object's true color is set by its absorption, yet it can look different under non-white light: a pure blue object under pure red light appears black. Under a single wavelength, every object might look like a shade of that one color, from bright to black. How life would adapt to that is speculation.

Each color of light bends a tiny bit differently in water and glass.

Violet bends the most. Red bends the least. So the colors spread apart into a rainbow.

When light goes from air into water or glass, it bends. Here's the trick: each color bends by a slightly different amount. Violet bends the most and red the least.

So when white light, which is all the colors mixed, passes through a raindrop or a prism, the colors come out pointing in slightly different directions and spread apart into a rainbow. If light had only one color, it would all bend the same way, and there would be nothing to spread.

Dispersion occurs whenever the propagation of light depends on wavelength. By Snell's law the angle of refraction depends on the refractive index, and for a given medium n varies with wavelength (OpenStax Table 1.2: crown glass 1.512 at 660 nm to 1.530 at 410 nm). White light is therefore fanned into a continuous spectrum.

OpenStax stresses that although we name about six colors, the rainbow is a continuous distribution of wavelengths. With monochromatic light the spread would be zero: refraction without dispersion.

You can make a rainbow with a glass of water and sunshine.

Scientists long ago used giant water drops to learn how rainbows work.

People have studied rainbows for a very long time. OpenStax describes how the scientist Kamal al-Din al-Farisi, working in Persia, filled a large glass vessel with water to act as a giant raindrop, and used it to work out that water droplets split white light into the colors of the rainbow. Theodoric of Freiberg, in Germany, did similar experiments around the same time.

You can see color mixing at work too. A blue toy under pure red light looks black, OpenStax explains, because it soaks up the red light and has no blue light to bounce back.

OpenStax credits Kamal al-Din Hasan ibn Ali ibn Hasan al-Farisi, working in Persia, with innovative experiments: lacking microscopes to study tiny droplets, he placed a large water-filled glass vessel inside a camera obscura and deduced that droplets decompose white light into the colors of the rainbow; his contemporary Theodoric of Freiberg did similar work.

OpenStax also explains secondary rainbows: two internal reflections produce a rare arc above the primary, with the color order reversed. With single-wavelength light, neither arc would have a spread of colors to show.