How Do Rainbows Form? How Sunlight and Water Make Colour

Sunlight looks white, but it contains many wavelengths of visible light. When light passes from air into water, its speed changes and the ray bends. This bending is called refraction. Different wavelengths bend by slightly different amounts, which begins to separate the colours.

Inside the droplet, some of the light reflects from the back surface. It then travels back toward the front of the droplet and refracts again as it leaves the water. By the time the light reaches your eye, the colours have spread out enough for you to see them separately. Red appears on the outer edge of a primary rainbow, while violet appears closer to the inner edge.

This sequence is the central answer to how rainbows form. A single droplet does not send a complete miniature rainbow into your eye from every direction. Instead, different droplets send different colours toward you at particular angles. Your brain combines light from huge numbers of droplets into one bright arc.

To see a primary rainbow, the Sun needs to be behind you and water droplets need to be in front of you. The rainbow forms around the point in the sky directly opposite the Sun. That geometry is why rainbows are often seen when a shower is moving away and sunlight breaks through behind the observer.

A rainbow is actually part of a circle centred on that opposite point. From the ground, the horizon usually blocks the lower part, so you see an arc. From a high viewpoint such as an aircraft, it is sometimes possible to see much more of the circle because fewer parts are hidden by the ground.

The exact droplets creating the rainbow for you are not the same droplets creating it for someone standing somewhere else. Move your position and the viewing geometry changes. This is a useful detail when learning how rainbows form because it explains why a rainbow has no fixed location that you can simply walk up to.

A secondary rainbow can form when light reflects twice inside a droplet before leaving. This extra reflection sends the light out at a different angle and reverses the colour order, so red appears on the inner edge and violet on the outer edge of the secondary bow. The extra reflection also loses more light, making the second rainbow fainter.

Large, nearly spherical droplets can produce bright, well-separated colours, while very tiny droplets can make the rainbow look paler. Sometimes extra faint bands appear just inside the main bow because light waves interfere with one another. These are called supernumerary bows.

When you revise how rainbows form, focus first on the three main processes: refraction, internal reflection, and dispersion. Once those are clear, the shape, colour order, and location of a rainbow follow naturally from the path that light takes through millions of droplets.

Sunlight refracts as it enters water droplets, reflects inside them, and refracts again as it leaves. Because different wavelengths bend by different amounts, the light spreads into colours. The geometry places the rainbow opposite the Sun, with countless droplets each contributing light to the arc you see.