A rainbow appears after the reflection and refraction of sunlight in raindrops. For a rainbow to appear, the Sun must be behind the viewer so that the light can reflect towards both the viewer and the raindrop.

The light is made of different colors, traveling at somewhat different speeds within the water droplets. The different wavelengths of light refracted by a rain droplet spread out, causing the arc of colors.

The color spectrum visible in a rainbow is caused by dispersion because different frequencies of light are bent by different amounts while passing through a raindrop. The result is that red light is separated from violet, with the intervening colors appearing between (orange, yellow, green).

Each color, therefore, represents a specific range of frequencies. Red light has the longest wavelength; violet has the shortest. When white light passes through another medium (liquid water or glass), its components may separate according to their wavelengths and be absorbed by that medium (chromatic dispersion). However, White light will also be divided into its composite colors by a diffraction grating. The effect is called chromatic diffraction.

The color spectrum contains violet, indigo, blue, green, yellow, orange, and red.

So whenever you see a rainbow, you can count precisely 7 colors. However, it’s impossible to distinguish between indigo and blue or between green and yellow with a naked eye.

Scientists have confirmed that the rainbow has 9 colors. Japanese researchers have seen a ‘ninth’ color in a rainbow for the first time, although human eyes cannot see it. The team, including Yoichi Aso and Shigeo Hayashi of Hokkaido University’s Department of Applied Physics, found that when white light is scattered into its spectral components, one of the colors that appeared wasn’t blue or violet as it should be. And, when they split up the rainbow into its constituent colors with a spectrometer, the color was nowhere to be found.

To create their ‘ninth’ color, the team used an ultraviolet laser with a wavelength of 355 nm to split sunlight up into its component colors. The laser produced what’s known as an auroral beam — light created by the scattering of electrons high up in Earth’s atmosphere. And when they directed this laser at a helium-neon gas jet, it separated another color that was not blue or violet.

Aso told New Scientist that the ‘ninth’ color, which he and his team can’t see themselves, is actually “very similar to indigo,” but not identical. He said that to find a better name for this new color, they will have to reexamine the correspondence principle — a hypothesis put forward by physicist Max Planck in 1921, claiming that electromagnetic waves and the human experiences created by them can be related.

In other words, they’ll have to find out if there’s a connection between this newly discovered color and the way humans perceive it.