How Do Raindrop Size And Shape Change Rainbow Colors

The size and shape of raindrops have a significant effect on the brightness, vividness, and even the types of colors we see in a rainbow. The ideal rainbow is produced by perfectly spherical drops.

Effect of Raindrop Size

• Medium Drops (e.g., 1-2 mm): These are ideal. They are large enough to separate white light (refraction) effectively but small enough that air resistance doesn't deform them. They remain almost perfectly spherical, producing bright, well-defined colors.
• Very Small Drops (e.g., < 0.1 mm, as in fog or mist): When droplets are tiny, an effect called diffraction (the bending of light waves as they pass the edge of the droplet) becomes dominant. Diffraction smears the colors, causing them to overlap and mix. This results in a very pale, wide rainbow with only faint colors, which is known as a "fogbow" or "white rainbow."
• Supernumerary Bows: Drops of a uniform, medium size (around 0.5 mm) can cause interference, where light waves exiting the drop interact with each other. This creates faint bands of pink and green just inside the primary rainbow's violet edge. These are called supernumerary bows.

Effect of Raindrop Shape

• Small to Medium Drops: As mentioned, these are held in a nearly perfect spherical shape by surface tension, which is the ideal shape for refraction.
• Large Drops (e.g., > 3 mm): Large raindrops fall faster. Air resistance (drag) becomes stronger than surface tension and flattens the bottom of the drop, making it look like a "hamburger bun." This non-spherical shape distorts the optical process. The light is no longer reflected and refracted at precise, uniform angles. This distortion causes the colors to blur together, making the rainbow less distinct. It particularly affects the top of the rainbow's arc, which can appear smudged or less colorful than the sides.