A rainbow forms when sunlight enters water droplets, bends, reflects inside, and bends again as it leaves. Different visible wavelengths emerge at slightly different angles. Millions of droplets then send separated colors toward an observer whose position, the Sun, and the droplets have the right geometry.
What happens in five steps
1. Sunlight approaches a droplet
The light that looks white contains a continuous range of visible wavelengths. A rainbow needs a bright source, usually direct sunlight, and many nearly spherical water droplets. Rain, spray, or mist can supply the droplets even when the air around the observer is dry.
2. The light refracts as it enters water
Light travels more slowly in water than in air. Unless it enters straight through the surface, that speed change bends the ray. The refractive index varies slightly with wavelength, so violet bends more than red. This first refraction begins to spread the colors.
3. Part of the light reflects inside the droplet
At the rear surface, some light passes out and some reflects back through the water. The familiar primary rainbow uses light that has reflected once. The droplet is not a tiny flat mirror; its curved surface sends different incoming rays along different paths.
4. The light refracts again as it exits
Returning from water to air changes the light’s speed again and bends the ray a second time. The combined refraction, internal reflection, and curved geometry concentrate outgoing light near particular angles. Red in the primary bow reaches an observer near an angle of about 42 degrees from the antisolar direction, while violet emerges at a slightly smaller angle.
5. Many droplets build the colored arc
One droplet sends only a narrow part of the spectrum toward one observer. Droplets higher, lower, left, and right supply different colors at the needed angles. Your eye combines light from this enormous collection into an arc with red on the outside and violet on the inside.
A rainbow belongs to the observer
A rainbow is not a colored object hanging at a fixed distance. Its center lies on the imaginary line extending from the Sun through your head toward the opposite side of the sky. Someone standing beside you receives light from a different set of droplets and therefore sees a slightly different rainbow. You cannot walk to its end because its viewing geometry moves with you.
The Sun generally needs to be behind you and droplets need to be ahead. A low Sun puts the bow high in the sky; a high Sun puts its center far below the horizon, leaving a smaller visible arc. Near local midday, the primary bow may be entirely below the horizon at many latitudes.
Why a rainbow is really a circle
All droplets that send a chosen color at the correct angle lie along a cone around the antisolar line. Where that cone appears projected against the distant shower, the result is a circle. The ground normally blocks the lower part, so viewers see an arc. From an aircraft or a high viewpoint with droplets below, a nearly complete circle can sometimes be visible.
How a double rainbow forms
A secondary rainbow comes from rays that reflect twice inside droplets before leaving. That extra reflection loses more light, so the second bow is fainter. It emerges at a larger angle, appears outside the primary bow, and reverses the color order: red lies on its inner edge and violet on its outer edge. The region between the bows can look darker because fewer rays leave droplets at those angles.
Why some rainbows look brighter
Large droplets tend to produce more distinct colors, while very small droplets allow wave effects to blur the bands into a pale fogbow. Bright sunlight, a dense curtain of drops, and a dark background increase contrast. Additional interference among nearby ray paths can produce faint pastel bands just inside the primary rainbow, called supernumerary bows.
Not every colored arc is a rainbow
Halos and sundogs usually come from refraction through ice crystals, not liquid drops. Iridescent clouds and the colors on a soap film depend strongly on interference or diffraction. A garden sprinkler can make a true small rainbow because spherical water droplets and the same Sun-observer geometry are present.
Three useful corrections
- A droplet does not send its complete rainbow straight into one eye; different droplets supply the visible colors.
- The seven named color bands are a convenient description of a continuous spectrum, not seven hard divisions in the light.
- A rainbow has no fixed physical distance or reachable endpoint.
A viewing safety boundary
Keep the Sun behind you when observing a rainbow and never turn optical equipment toward it without a certified solar filter fitted for that instrument. Wet roads, cliffs, shorelines, thunderstorms, and roadside stops can create more immediate hazards than the optics. Observe from a secure location.
The useful mental model
A raindrop is a curved light sorter. It bends sunlight on entry, reflects part of it, bends it again on exit, and sends one narrow color range toward you. A sky full of droplets assembles those contributions into a circle.
