The Physics of Rainbows
A rainbow is sunlight dissected by millions of raindrops, each acting as a tiny prism-and-mirror. The bow you see is geometry: every drop sends its brightest light to your eye at exactly 42° from the antisolar point.
Refraction + dispersion
Sunlight entering a droplet refracts, and because water's refractive index varies with wavelength (dispersion), violet bends more than red — white light fans into a spectrum.
Internal reflection
The light reflects once off the droplet's inner back surface, then refracts out toward your eye. The primary bow's 42° radius is pure geometry from Snell's law applied twice plus one reflection.
The reversed secondary bow
Drops also send light that reflected twice inside — fainter, at 51°, with colors reversed (red inside, violet outside) because the extra reflection flips the order.
Supernumerary bows
Faint extra bands inside the primary bow come from wave interference between rays taking slightly different paths — Thomas Young used them as early evidence for light's wave nature.
Try this yourself
- Spray a garden hose on a sunny day with the sun behind you — walk until the bow appears at 42°.
- Look for the fainter secondary bow outside the main one and check its reversed colors.