Rainbow

A rainbow is a natural optical phenomenon that appears when sunlight interacts with water droplets in the atmosphere. The colors usually seen in a primary rainbow are red, orange, yellow, green, blue, and violet, arranged from the outer edge toward the inner edge. Sunlight may look white, but it contains a continuous range of visible wavelengths. When sunlight enters a raindrop, it is refracted, meaning its direction changes as it passes from air into water.

Different wavelengths are refracted by slightly different amounts, separating the light into its component colors. The light then reflects from the inside surface of the droplet and is refracted again as it leaves. Millions of droplets can send these separated rays toward an observer, creating the curved band we recognize as a rainbow. The colors do not actually exist as separate stripes in the sunlight; the boundaries we see are created by our visual system and the continuous spectrum of light.

A rainbow is not a physical object located at a particular place in the sky. Its appearance depends on the position of the Sun, the water droplets, and the observer. For a primary rainbow, sunlight generally needs to come from behind the observer while rain or fine water droplets are in front of them. The rainbow forms a circle around the point opposite the Sun, although from the ground we usually see only part of that circle because the horizon blocks the lower portion.

From a high position, such as an aircraft, a much larger part of the circle can sometimes be visible. A secondary rainbow can also occur when light undergoes two internal reflections inside the droplets. It appears outside the primary rainbow, with its colors reversed and usually less bright. Rainbow colors therefore illustrate how refraction, reflection, the wavelength of light, and the geometry between the Sun, droplets, and observer combine to create a familiar visual phenomenon.