Refraction ‒ Rainbow

Rainbow is an optical phenomenon manifested as the color group of concentric arcs, generated by refraction and internal reflection of sunlight on water droplets in the atmosphere. During a rain or a fog the light passes through every single drop. Because water has a higher refractive index than air, the light is refracted therein. The refractive index is different for different wavelengths of light and the surface of the droplets has a spherical shape. At the surface of raindrops the light therefore extends to color components, reflects on the inner surface and leaves the droplet at various angles. Drops, which are at the same angular distance from the light source (the Sun) will then appear as if they were of the same color. Therefore, the rainbow is circle shaped or as a part of a circle. In the raindrop not only one, but multiple reflections of the light beam can occur. When two internal reflections occur, a secondary rainbow can be observed. The secondary rainbow appears above the primary rainbow and the sequence of colors in the secondary rainbow is opposite to the primary rainbow. The secondary rainbow is also usually much weaker than the primary rainbow.