A rainbow forms when sunlight interacts with tiny water droplets in the air, usually after rain. This phenomenon depends on the observer's position in relation to the sun and the droplets. The result is a multicoloured arc in the sky, seen only if the sunlight is behind the observer and there are droplets suspended in front.
The phenomenon begins with sunlight
Everything starts when sunlight meets a region with water droplets suspended in the air, as happens right after rain. These droplets act as small natural prisms, separating white light into several colours. The effect depends not only on the droplets, but also on the quality and intensity of the light at that moment.
Besides rain, a rainbow can appear near waterfalls, fountains or even the mist formed by sea waves. In these situations, the small water particles allow light to be broken up in a similar way to after rain. The phenomenon, however, is easiest to see on partly cloudy days with the sun low on the horizon.
For the arc to be visible, you need to be in a position opposite the sun, usually with it low in the sky. The lower the sun, the larger the rainbow you can observe. In general, the arc appears early in the morning or late in the afternoon, times favourable to its formation.
White light and the separated colours
The light that comes from the sun looks white but is actually made up of several mixed colours. When this light enters a water droplet, it is separated by a process called dispersion. Each colour bends at a different angle, forming the famous coloured arc.
The order of the colours always follows the same pattern: red, orange, yellow, green, blue, indigo and violet. This happens because each colour passes through the water differently, resulting in the visible bands. Red bends the least and appears at the top; violet, bending the most, sits at the base of the arc.
When there are many droplets in the air, each one contributes to forming a part of the arc. Our eye sees the sum of all these small individual arcs, aligned according to the angle of incidence of the light. The result is the complete rainbow, visible for a few minutes.
How raindrops take part in its formation
Each raindrop acts as a tiny prism, responsible for breaking and reflecting the light. The secret lies in the size of the drop: medium droplets produce sharper coloured arcs, while with very small droplets the arc may look faded. In rare cases, a secondary arc appears too, less intense and with inverted colours.
Not every rain forms a rainbow, since the density and distribution of the droplets must be suitable. Areas with many scattered tiny droplets make the phenomenon easier to form. In heavy rain with large, close droplets, the rainbow may not form or may be barely visible.
- Medium droplets create a bright arc with striking colours.
- Very small droplets leave the colours soft and barely noticeable.
- Even distribution of droplets enhances the sharpness of the arc.
- Sprays of water can also form mini rainbows.
- A double arc occurs when light reflects twice inside the droplet.
The role of the observer's position
Seeing a rainbow depends entirely on where you are in relation to the sun and the water droplets. Whoever has the sun behind them, looking at the rain in the distance, has the ideal angle to see the phenomenon. If the observer changes position, the point where the arc appears also changes, since each person sees "their own" rainbow.
For this reason, two people looking at the same region of the sky see the rainbow in the same place, but technically they are observing different sets of droplets. This happens because the angle between the sunlight, the droplet and the eye must be about 42 degrees for the main arc.
If you are in an airplane, you can see a complete, circular rainbow, with the ground not getting in the way. On the surface, we see only a segment of this circle because of the horizon. This detail reinforces that everything depends on perspective and position.
How a rainbow forms: main steps
- Sunlight strikes a water droplet, undergoing refraction as it enters.
- Inside the droplet, the light is reflected off the inner surface, reversing its direction.
- It crosses the edge of the droplet again, undergoing a new refraction as it exits.
- The colours separate due to dispersion, forming a coloured spectrum.
- The human eye only perceives the arc when the light reaches it at a certain angle.
- Adding up the light from many droplets forms the rainbow visible in the sky.
What can change the appearance of a rainbow
The width of the arc, its sharpness and intensity vary due to factors such as droplet size and light conditions. More visible rainbows appear with medium droplets and strong sun. With little light or tiny droplets, the phenomenon becomes faint or hard to notice.
Location also matters: in cities, excess pollution can make clear forms difficult, while open areas tend to favour clearer appearances. Time of day is decisive; only with the sun low does the arc have room to form the characteristic semicircle along the horizon line.
In rare situations, it is possible to observe a double rainbow or a weaker phenomenon called a white rainbow, when the colours barely appear. The main explanation for this variety lies in the mix of droplet size and the intensity of the light that reaches them.
A common mistake about the rainbow
Many people think it is possible to find the end of a rainbow, as legends often suggest. In practice this does not happen, because the rainbow does not exist in a fixed place in space. It depends on the viewing angle between each observer, droplet and light source.
Another mistaken idea is that there will always be a rainbow whenever it rains and the sun shines. Although it may seem common, several conditions must be met, such as the position of the sun, the distance of the droplets and atmospheric conditions, for the phenomenon to be seen.
Many people believe the rainbow is just a semicircle, but it can actually form a complete circle when seen from above. From the surface, we only see part of it because the ground blocks its continuation. This highlights the role of the environment and perspective in the phenomenon.
Frequently asked questions
Why does the rainbow sometimes have more vibrant colors?
The brightness of rainbow colors depends on the size of the water droplets and the intensity of the sunlight present. Medium-sized droplets and strong light produce more striking and vibrant colors. When there are many small droplets or low luminosity, the colors appear diluted and barely noticeable.
Is it possible to see a rainbow anywhere?
Rainbows can be seen anywhere in the world where there is sun and water droplets in the air, but visibility depends on favorable conditions. Open places with little pollution make the phenomenon easier to see, while large cities can make it harder due to pollution or buildings.
Is there a difference between a rainbow seen from the ground and from a plane?
From the ground, we only see a segment of the rainbow, since the horizon blocks part of the circle. From an airplane or at great heights, it is possible to see an almost complete circle, as there are no visual obstacles interrupting the view of the phenomenon.
What is a secondary rainbow?
A secondary rainbow occurs when light is reflected twice inside the water droplets, forming a second arc outside the main one. Its colors appear inverted and are always fainter, since part of the light scatters before reaching the observer's eyes.
The beauty of light unfolding in the droplets
The rainbow is a fascinating example of how physics and nature combine to create surprising images in everyday life. It reveals how simple details, such as droplets and light, can form a spectacle full of colors, depending only on our perspective and the environment.
Understanding this phenomenon makes its appearance even more special, showing that there is science and poetry in every colorful outline in the sky. When we observe the rainbow, we see not only an optical effect, but a reminder that small interactions turn the ordinary into something admirable.
