Rainbows develop when sunlight passes through falling raindrops. As that light passes through water, however, it undergoes scattering. By bouncing off of particles in the water, that light no longer takes a straight, unimpeded path. Its intensity, too, can drop as some of the light is absorbed. Physicists refer to these changes as attenuation (Ah-ten-yu-AY-shun). That same thing happens when sunlight shines through a raindrop.

Rob Hart is the lead meteorologist for the National Weather Service in Charleston, W.V. “Sunlight actually has all kinds of colors in it,” he explains. “When the sunlight passes through raindrops, the water makes the sunlight bend.” Scientists refer to that bending as refraction.

Because each hue has a slightly different wavelength, each refracts a different amount. That refraction separates the colors and sends them out of the raindrop heading in slightly different directions. This transforms the sun’s light into a gorgeous arc across the sky.