What is a rainbow color

How is a rainbow created?

If the sun comes out after a rain shower, a rainbow often forms in the sky. The colorful play of light can be explained with the basics of optics.

At first glance, the light of the sun usually appears white. In fact, it is made up of a wide range of wavelengths. This becomes visible in a rainbow in which the sunlight fans out into its different color components - from red to blue. This phenomenon is due to an effect that also occurs in a prism: As soon as electromagnetic waves pass from one medium - such as air - into another medium - such as the glass of the prism - the light rays are refracted at the interface. “The deflection is different for different wavelengths,” explains Michael Vollmer from the Technical University in Brandenburg.

This effect can also take place in the atmosphere when rays of the sun hit the tiny water droplets in a rain front. "When entering the raindrop, the sunlight splits into a color spectrum like a prism," says Vollmer. This splitting of the sunlight is only the first step in the formation of a rainbow.

Ray path in the water drop

Part of the incoming rays of the sun crosses the roughly spherical raindrop and leaves it at the rear, where it is deflected again. Another part, however, is reflected on the curved rear side and comes out again on the front side. The light is also refracted again at the interface between water and air. The angle at which a ray of light emerges from the raindrop depends on the angle of incidence. However, due to the geometry of the drop, a particularly large number of light rays are deflected equally and can be observed at an angle of around 42 degrees. “This phenomenon creates the shape of the rainbow,” reports Vollmer. And the well-known color gradient in the sky results because the observation angle for blue colors at 40 degrees is about two degrees smaller than for red colors.

For a rainbow, of course, you don't need just one, but countless raindrops that break and reflect the sunlight. We also have to be in the right position to be able to see the natural spectacle, since the raindrops do not reflect the same amount of light in every direction. “The most light reaches our eyes when the angle between our line of sight to the raindrop and our line of sight to the opposite point of the sun is about 42 degrees. All drops that meet this condition form an arc of a circle - the rainbow, ”says Vollmer. Strictly speaking, this light can even form a whole circle. “But mostly the landscape cuts off the lower part of the rainbow. With a little luck you can watch the whole circle from an airplane or a high mountain. "

Creation of the arch form

On some days, a second rainbow can even be seen in the sky just outside the first. “The light that forms this secondary rainbow was reflected again within the raindrop and only then left the raindrop,” says Vollmer. Theoretically, the light can be reflected even more often within the raindrop, but it loses intensity in the process. In addition, the position of the arcs changes: the arcs of the third and fourth order are not opposite, but in the direction of the sun. Therefore, with increasing order, rainbows can no longer be seen with the naked eye, even in excellent visibility conditions.