What causes rainbows?

Rainbows are caused by the interaction of sunlight with water droplets in the atmosphere, such as rain, mist, or spray. When sunlight enters a water droplet, it slows down and bends—a process called refraction—because light travels at different speeds in air and water. Inside the droplet, the light reflects off the inner surface and then refracts again as it exits the droplet, bending once more as it moves back into the air. This combination of refraction and internal reflection causes the sunlight to spread out into its component colors, creating a spectrum. Each color bends by a slightly different amount due to its wavelength, with red bending the least and violet the most. This separation of colors produces the circular arc of a rainbow, typically seen as red on the outer edge and violet on the inner edge. The specific angle at which light exits the droplet to reach the observer’s eye is about 42 degrees for the primary rainbow, which is why rainbows appear at a certain angle relative to the position of the sun and the observer. Secondary rainbows can sometimes appear outside the primary arc, caused by light reflecting twice inside the droplets, resulting in reversed color order and a dimmer appearance. In short, rainbows are the result of sunlight being refracted, reflected, and dispersed by water droplets, creating the beautiful multicolored arcs we see in the sky.