A rainbow is an optical phenomenon, an effect seen when light interacts with water droplets in a specific way. It does not automatically appear after every rain shower, as its visibility depends on a precise combination of atmospheric conditions and the observer’s position. This spectacular arc of color is fundamentally the result of sunlight being scattered and separated by airborne moisture.
The Essential Ingredients for a Rainbow
The formation of a rainbow requires two elements: direct, bright sunlight and a suspension of water droplets in the atmosphere. The water does not have to be an active rain shower; mist, fog, ocean spray, or even a garden sprinkler can provide the necessary airborne moisture. These droplets act as tiny, spherical prisms.
The sun must be shining brightly enough to cast shadows. If the sky is completely overcast, no rainbow can form because the light is too diffuse and scattered. Therefore, the familiar phrase “rain and sun” is more accurate than just “after rain,” as both must be present simultaneously in the viewing area.
Why Viewing Angle Determines Visibility
A rainbow is only visible when the observer is positioned with the sun directly behind them, shining onto the water droplets in front. The center of the rainbow’s perfect circle, a point called the anti-solar point, is always directly opposite the sun and aligned with the observer’s shadow. This geometric arrangement means the rainbow appears to move as you move, as it is unique to your line of sight.
The light that creates the primary rainbow is reflected back toward the observer at a specific angle of approximately 42 degrees from the anti-solar point. All water droplets that reflect light at this 42-degree angle form a cone, which creates the visible arc. If the sun is higher than 42 degrees above the horizon, the entire circular rainbow is cast below the horizon, making it invisible from the ground. Rainbows are most often seen in the morning or late afternoon when the sun is low in the sky.
How Light Separates into Color Bands
The vibrant color bands are created by a three-step process. Sunlight first enters the drop, causing it to slow down and bend, a process called refraction. The light then travels to the back of the droplet, where it reflects off the interior surface.
Finally, the light is refracted a second time as it exits the droplet. This bending separates the colors because the degree of refraction, known as dispersion, depends on the light’s wavelength. Shorter wavelengths, such as violet light, bend slightly more than longer wavelengths, like red light. This difference in bending angles causes the white sunlight to fan out into the familiar spectrum of colors before it reaches the eye.