A rainbow appears when sunlight interacts with water droplets and some of that light reaches your eyes along particular directions. Refraction bends the light as it enters and leaves a droplet, while internal reflection redirects part of it. Because different wavelengths bend by different amounts, the returning light becomes separated into colors.
The result depends on the Sun, the droplets and the observer together. A rainbow is not a colored object fixed above a particular field or mountain. It is an optical pattern you see from your position, which is why its apparent endpoint cannot be reached by walking toward it.
Follow light through one droplet
Imagine sunlight arriving at a round water droplet. At the air-water boundary, the light changes speed and, except at special angles, changes direction. This is refraction. Some entering light travels across the droplet and reflects from its inner surface before heading back toward the front.
When that reflected light leaves the droplet, it crosses the boundary again and is refracted a second time. The primary rainbow involves one internal reflection between those two boundary crossings. Other light takes other paths, so a droplet does not send all the sunlight striking it into the rainbow.
The Met Office describes this sequence as the basic mechanism behind the familiar bow. It is worth keeping the steps separate: refraction bends light at the boundary, reflection changes its direction inside, and the exit refraction helps determine where it travels afterward. No colored material needs to be present in the rain.
White sunlight contains many wavelengths
What looks like white sunlight includes a broad range of visible wavelengths. Water's refractive index varies with wavelength, so violet light is bent more strongly than red light under the same entry conditions. This wavelength-dependent behavior is called dispersion.
The colors are therefore already represented in the incoming sunlight. The droplet sorts their outgoing directions rather than manufacturing red, green or blue. In the primary bow, red appears along the outer edge and violet toward the inner edge.
The familiar list of seven colors is a useful naming tradition, but the spectrum is continuous. There are no physical borders at which one named color suddenly becomes the next. How many bands appear distinct also depends on brightness, the background, the observer's vision and the image or screen used to display a photograph.
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Why the colors make an arc
A primary rainbow is brightest near a particular angular distance from the direction exactly opposite the Sun. That opposite direction is called the antisolar direction. The red outer edge is roughly 42 degrees away from it; shorter-wavelength colors appear at slightly smaller angles.
All viewing directions at the same angle around that center make a circle. When illuminated droplets occupy those directions, they can contribute to the bow. The shape follows the geometry of light reaching the observer, rather than the shape of a curved band of rain hanging in the sky.
For a ground-level observer, the lower part of this potential circle is commonly hidden by the ground or lacks suitable visible droplets. That leaves an arc. A lower Sun generally gives a taller visible bow above the horizon. From an elevated viewpoint, with droplets below as well as ahead, more of the circle may become visible.
Each observer has a different view
The Hong Kong Observatory explains that different colors reaching an observer come from different sets of droplets. A single droplet sends light in several directions, but it contributes the appropriate wavelength to your eye only when its position and the light path align.
Move a few steps and a different collection of droplets supplies the pattern you see. A person standing beside you may describe the same apparent rainbow over the same landscape, yet the precise rays reaching their eyes differ from yours. Even a camera records the pattern from the position of its lens.
This is the reason a rainbow has no reachable endpoint. The apparent meeting with a tree or hillside is a line-of-sight alignment. Walking to that tree changes your observing position rather than carrying you to a solid colored structure. The water droplets are real; the arc is a perspective-dependent arrangement of the light they send to you.
A second reflection creates a second bow
A secondary rainbow involves two internal reflections before the light leaves the droplet. The additional reflection produces a different outgoing geometry. The secondary bow appears farther from the antisolar direction than the primary, so it usually looks like a larger arc outside it.
Its colors run in the opposite order: red is on the inner side and violet on the outer side. The extra reflection also reduces the light reaching the viewer along this path, which helps explain why the secondary bow is usually fainter.
NOAA's educational explanation distinguishes the two paths rather than treating the second bow as a mirror image cast onto a cloud. Looking for reversed color order is a good way to identify a genuine secondary rainbow in a photograph. Two bright bands do not automatically mean two separate rainbows, especially when faint supplementary bands or image-processing effects are involved.
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Rain is only one source of droplets
Waterfalls, fountains and sprinkler spray can provide the same essential ingredient: droplets illuminated by light from the correct direction. The water does not need to be falling from a raincloud. This is why a rainbow can appear near a waterfall on an otherwise clear day.
OpenStax emphasizes that the observed pattern depends on light from numerous drops reaching the eye. Spray continually changes as droplets move through the right positions, but the overall arc can appear steady while the illumination and viewpoint remain suitable.
The waterfall photographs here show this relationship clearly. The colored arc belongs to the sunlit spray, not to a permanently colored section of the rock or falling water. A visitor standing elsewhere on the same trail may see a different part of the bow or no bow at all. A photograph preserves one observer's alignment at one moment.
When and where to look
A useful starting point is a low Sun behind you and illuminated rain or spray ahead. Morning and late afternoon often provide favorable geometry for a bow above the horizon. The Bureau of Meteorology also notes the value of gaps in clouds that allow sunlight to reach the droplets.
Dark clouds behind the droplets can make the colors easier to see by improving contrast. They are a helpful background, not a required ingredient. A rainbow can fade when sunlight is blocked, when the droplets move away or when your own viewpoint no longer lines up with the visible spray.
The often-quoted 42-degree limit concerns the primary bow above a level horizon. It is not a ban on seeing rainbows when the Sun is higher: an observer looking downward into spray can have suitable droplets below the horizon. The more general rule is the relationship among illumination, droplets and viewing direction.
Make a safe observation record
Observe from a sheltered or otherwise safe place. A rainbow after a shower does not prove a storm is harmless. If thunder is audible, go to a substantial building or an enclosed, metal-topped vehicle with the windows closed rather than remaining in an exposed location for a photograph. Stay sheltered for at least 30 minutes after the last thunder. At waterfalls, stay on designated routes and behind barriers; wet rock can be extremely slippery.
For a simple notebook entry, record the time, the direction of the Sun behind you, the source of the droplets and whether a second bow is visible. Note where the red edge sits. If the rainbow changes, write down whether the lighting, rain or your position changed too.
A phone camera is enough to document the geometry. Include some surrounding landscape for context, but remember that a hill appearing beneath the bow is not necessarily where the contributing droplets are located. Keep editing modest when using the image to explain color order. Strong saturation can make a dramatic picture while obscuring what was actually visible.
Frequently asked questions
Is a rainbow made by reflection or refraction?
Both are involved. The primary bow comes from light refracted into a droplet, reflected once inside and refracted out again. Dispersion during refraction separates the outgoing directions of different wavelengths.
Why is red on the outside?
The primary rainbow sends red light toward the observer at a slightly larger angle from the antisolar direction than violet light. That geometry places red along the outer edge of the primary arc.
Can a rainbow be a complete circle?
Yes, when the viewpoint and illuminated droplets allow the full set of directions to be seen. Ground-level views usually hide or lack the lower portion, leaving only an arc.
Why can two people not reach the end together?
There is no fixed physical endpoint to reach. Each person's rainbow is defined by the light paths to their own eyes. Moving changes those paths, even if the bow still seems aligned with a recognizable feature in the landscape.
Does a double rainbow mean two rain showers?
No. The main distinction is the number of internal reflections involved in the light paths. The same field of illuminated droplets can produce both primary and secondary bows for an observer.
Sources and further reading
National Weather Service: rainbow geometry
NOAA NESDIS: color and secondary rainbows
OpenStax: dispersion and rainbows
Hong Kong Observatory: colors from different droplets
Australian Bureau of Meteorology: rainbow viewing conditions
National Weather Service: lightning safety
Photo credits
A bright primary rainbow and fainter secondary rainbow arc above a landscape: Erwin Van Pottelberge; CC BY-SA 3.0. Reproduced without editorial alterations; display size may be optimized.
A double rainbow appears in sunlit spray at Skógafoss waterfall: Austaclausen; CC BY-SA 4.0. Reproduced without editorial alterations; display size may be optimized.
Rainbows curve through spray below Vernal Fall in Yosemite: Wing-Chi Poon; CC BY-SA 2.5. Reproduced without editorial alterations; display size may be optimized.
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