Read also: Why does the sky turn red at sunset? ↗
Read also: What are clouds made of, and why do their shapes differ? ↗
A rainbow needs light, droplets and an observer
A rainbow forms when light passes through water droplets, changes direction and returns towards an observer’s eyes with its colours partly separated. In the usual case, the source is the Sun behind the observer, while the droplets are in front. It is neither a coloured stripe painted on a cloud nor an object suspended at a fixed location.
The viewer’s position is part of the phenomenon. Water and sunshine can be present without a rainbow being visible from a particular spot. The droplets must receive light and send it towards the eyes at suitable angles. An opening in cloud can therefore suddenly reveal a bow over a landscape that, moments earlier, looked simply rainy.
White light already contains the colours we see
Sunlight contains a distribution of visible wavelengths that, together, we perceive as approximately white. Water does not manufacture the rainbow’s colours as though it contained paint. It allows components already present in the incident light to travel in partly separated directions.
Wavelength is one way to describe light: within the visible range, red corresponds to longer wavelengths and violet to shorter ones. These components do not undergo exactly the same deviation while passing through a droplet. Comparing it with a prism helps explain separation, but a drop has its own shape and internal path. A rainbow also requires reflection within the water and the right geometry between the light source, droplets and observer.
Refraction, reflection and dispersion are different ideas
Light changes speed and direction when it enters water from air: this is refraction. Some light reaches the droplet’s inner surface and is reflected. On emerging into air again, it undergoes another refraction. The path responsible for the primary bow includes one internal reflection between entry and exit.
Dispersion is the difference in deviation between wavelengths. Refraction thus describes a change of trajectory, reflection the return of part of the light, and dispersion helps explain colour separation. They are related aspects rather than synonyms. Not every ray striking a drop follows a rainbow path: some passes through in other directions and some is reflected immediately from the outer surface. The familiar bow selects particular routes from this larger collection.
Each droplet does not draw an entire little rainbow
Rain contains many drops in different positions. Light of a particular colour reaching your eyes comes from drops momentarily located in a suitable direction for that colour. Together they create a continuous band. One drop can send components in different directions, but it does not project a complete miniature bow at every point in the sky.
Imagine a curtain of water: numerous drops contribute simultaneously to different parts of the view. As they fall, other drops replace them in positions capable of sending light along the same direction. The bow can appear steady while its water keeps moving. Distinguishing the drops’ motion from the light pattern explains why the rainbow does not fall to the ground with the rain.
Why the primary bow appears at roughly 42 degrees
The primary rainbow has an angular radius of approximately 42 degrees at its red edge, measured from the direction opposite the Sun. Other colours occur at slightly different angles, with violet further inside. This describes a viewing angle, not a distance in metres to a wall of colour.
Think of the direction in which your shadow points: it is opposite the Sun. Around that direction, paths bringing rainbow light to your eyes form a cone whose tip is at the observer. The visible set of these directions creates a circle. Its apparent size follows this geometric relationship even though the drops can be nearby in a spray or much further away in a rain shower.
Why the Sun is normally behind the observer
For the usual primary rainbow, look towards illuminated water opposite the Sun. Looking at a grey cloud is not enough: sunlight must reach droplets occupying the appropriate directions. A shower ahead with clear sky behind can provide suitable conditions. Conversely, cloud covering the Sun can quickly remove the illumination required.
When the Sun is low, a bow can stand high above the horizon. With a higher Sun, more of its circle falls below the normal ground-level view. This explains why striking rainbows are often seen in the morning or late afternoon without making a clock time an absolute rule. Terrain, the observer’s height and the distribution of water also influence which part of the pattern becomes visible.
The complete rainbow is a circle
We usually see an arc from the ground because the landscape and the lack of droplets in lower directions limit what is visible. The underlying geometry nevertheless describes a circle around the point opposite the Sun. An elevated viewpoint, with water in the necessary directions, can reveal more of the figure.
NASA presents photographs of complete circular rainbows taken from the air. There is no need to imagine a different type or an added lower half: it is the same phenomenon under more favourable viewing conditions. Not every flight reveals a circle, because sunlight, water and direction must still coincide. Being high above the ground offers a possibility, not a guarantee of a particular view from the aircraft window.
Why a double rainbow reverses its colours
The usual second bow appears outside the primary one and is generally fainter. Its light undergoes two internal reflections within droplets instead of one. It emerges in a different direction, creating a bow with a larger angular radius and a reversed colour sequence.
In the primary bow, red lies at the outer edge and violet at the inner edge; in the secondary, that arrangement is reversed. It is not merely the first rainbow reflected on a cloud: different internal light paths are involved. The region between them can look darker and is called Alexander’s band. Its visibility depends on the conditions and contrast, so it need not appear sharply defined every time two bows are present.
Are there really seven separate colours?
Red, orange, yellow, green, blue, indigo and violet are the traditional seven named colours. This is a convenient way to memorise their order. The transitions are continuous, however: there are no seven compartments with rigid physical boundaries, nor an abrupt switch from green to blue like painted stripes in a drawing.
The Met Office notes that Newton popularised this seven-part division. Languages, visual habits and observing conditions can support other ways of grouping shades without changing the physics. If you cannot pick out indigo, it does not mean you saw an incomplete rainbow. Brightness, contrast, droplet size and the sensitivity of the eyes influence how distinct different parts of the coloured band appear.
Why you cannot reach the end of a rainbow
A rainbow does not have an end resting in a field or behind a house. Its apparent meeting point with the landscape is a viewing direction within an image. Moving changes which droplets can send light towards you along the angles required to form the bow.
Walking towards a rainbow is therefore different from approaching a bridge. Two people standing apart may point to a very similar figure, but each receives light according to their own viewing geometry. The difference can be unobvious when the bow is distant. In a small nearby water spray it is easier to notice how the pattern responds to changes in position and direction of view, rather than remaining attached to a fixed piece of ground.
Why a rainbow can fade or disappear
A strong bow needs enough illumination, droplets in the appropriate directions and sufficient contrast to distinguish its band. When cloud covers the Sun, a shower moves or the observer changes position, one of those conditions can be lost. The rainbow disappears without the water necessarily disappearing as well.
Droplet size affects its appearance too. In very small fog droplets, diffraction becomes important and colours are less distinct, producing a pale or whitish fogbow. This helps explain why optical phenomena do not always resemble saturated drawings. A photograph may also render shades differently from direct vision because of exposure, processing and the background’s contrast. The camera records light under its own settings, rather than preserving every aspect of human perception unchanged.
A simple, safe way to explore the geometry
On a sunny day, a fine spray of watering water can produce a small bow when illumination and position are suitable. Keep the Sun behind you and watch the water in front, moving slowly to see the figure appear or disappear. You do not need to look towards the Sun, use a lens or create a powerful jet.
Choose a safe, non-slippery surface and avoid wetting electrical equipment or wasting water. Do not remain outside searching for a rainbow during a thunderstorm. The observation connects three questions: where is the light source, where are the droplets and where is the observer? Their relationship explains the colours and shape more clearly than imagining a coloured object placed in the sky.
Frequently asked questions
What is needed for a rainbow to appear?
A light source, normally the Sun, illuminated water droplets and an observer in a suitable position. For the usual primary bow, the Sun is behind the viewer and the water ahead.
Why do the colours have that order?
Different wavelengths are not deviated by exactly the same amount. In the primary bow, its geometry places red at the outer edge and violet at the inner edge.
Does a rainbow have exactly seven colours?
Its band is continuous and contains many shades. The seven-colour sequence is a useful traditional division, not seven physically separate compartments.
Why are the second bow’s colours reversed?
A secondary rainbow involves two internal reflections within droplets rather than one. Light emerges in a different direction with the colour sequence reversed.
Can you see a round rainbow?
Yes. Its geometry is circular; from the ground, only a portion is usually visible. An elevated position can reveal the full circle if sunlight and droplets occupy the required directions.
Can you reach a rainbow’s end?
There is no physical end resting on the ground. As you move, the geometry and droplets sending light towards your eyes change.
Why is a rainbow easier to see with a low Sun?
With a low Sun, more of the primary circle lies above the normal horizon. The observer’s height and the distribution of droplets matter as well.
Does rain have to be falling where I stand?
No. Illuminated droplets are needed in the direction you observe. You can stand somewhere dry while seeing a bow in rain further away, or see a small one in a water spray.
