How does Snell’s Law apply to the formation of rainbows?

Snell’s Law plays a crucial role in the formation of rainbows by explaining how light bends (refracts) when it enters and exits raindrops.

1. Refraction on entry:
   When sunlight enters a spherical raindrop from the air, it moves from a medium with a lower refractive index (air) to a higher one (water). According to Snell’s Law, the light bends toward the normal, and the amount of bending depends on the light’s wavelength. Different colors (wavelengths) bend by different amounts—violet bends more than red.
2. Internal reflection:
   After refraction, the light hits the inner surface of the raindrop. Some of this light undergoes internal reflection, bouncing off the back of the drop and traveling back toward the front.
3. Refraction on exit:
   As the light exits the raindrop and re-enters the air, it moves from a higher to a lower refractive index. Again, Snell’s Law governs this bending—now away from the normal. The separation of colors becomes more pronounced at this stage.
4. Dispersion and angle formation:
   Because each color bends differently, the exiting rays spread out into a spectrum. The light that emerges at a specific angle (about 42 degrees for red and 40 degrees for violet) creates the circular arc we see as a rainbow.

Thus, Snell’s Law explains how light rays entering and exiting raindrops are refracted and dispersed, producing the colors and shape of a rainbow.