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The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of about 40-42 degrees. The different colours (wavelengths) of light have different refractive indices which cause them to bend unequally. Thus, a rainbow is arc-shaped.