What is the scientific explanation of a rainbow? — RS, Salinas, CA
A rainbow is caused by three important optical effects: reflection, refraction, and dispersion, all working together. The rainbow forms when sunlight passes over your head and illuminates falling raindrops in the sky in front of you. This sunlight enters each spherical raindrop, partially reflects from the back surfaces of the raindrop, and then leaves the raindrop and heads toward you. The raindrop helps some of the sunlight make a near U-turn. But the path that the light follows after it enters the raindrop depends on its color. Light bends or “refracts” as it changes speed upon entering water from air and the amount it bends depends on how much its speed changes. Since violet light slows more than red light, a phenomenon called “dispersion,” the violet light bends more than the red light and the two colors begin to follow different paths through the drop. All the other colors are spread out between these two extremes.
The colored rays of light then partially reflect from the back surface of the raindrop because any change in light’s speed also causes partial reflection. Now the various colors are on their way back toward you and the sun. The light bends again as it emerges from the raindrop and the various colors leave it traveling in different directions. Only one color of light will be aimed properly to reach your eyes. But there are other raindrops above and below it that will also send light backward and some of that light will also reach your eyes. But this light will be a different color. What you see when you observe the rainbow is the lights that many different raindrops send back toward your eyes. The upper raindrops send their red light toward your eyes while the lower raindrops send their violet light toward your eyes. You see a series of colored bows from these different raindrops.