Rainbows in Different Refractive Indices

Abstract

The rainbow is a natural optical scattering and dispersion phenomenon that reveals the visible spectral composition of sunlight in the shape of an arc. People are instinctively attracted by its colorful appearance and curved shape. Hence, there are many serious studies about the rainbow with a long history. Recently, several simple experiments, adopting glass balls, acrylic spheres, spherical flasks, or sessile water drops, have been devised to demonstrate how the rainbow is formed. These works demonstrate the colors and shapes of the rainbow well and explain how the dispersive spectrum is produced by the refraction–reflection–refraction process. However, the influence of the refractive index is rarely illustrated. It is not difficult to see that the refractive index of raindrops and the atmosphere is closely related to the rainbow, especially the viewing angle of it. In this paper, we use spherical lenses with different materials and in different solutions to change the refractive index. Under a collimated light source, the evolution of the viewing angles of primary and secondary rainbows with respect to the refractive index is demonstrated. Experiments with refraction conditions similar to a natural rainbow are also conducted.