August 30, 2023
3-5 minutes

What is a rainbow?

A rainbow is simply defined as an arc of colors (red, orange, yellow, green, blue, indigo and violet) that can be seen crossing the sky, usually from one end of the horizon to the other. Rainbows can be seen all over the world, and have been anchored in culture and myth for centuries. However, despite the tangible, historical and almost magical aspect of rainbows, they can be considered a meteorological phenomenon. Simply put, when light reflects off atmospheric water particles, a temporary reflection is created in the sky, which can only be seen under the right conditions and in the right place. A rainbow is essentially a meteorological optical illusion.

Rainbow formation

Basic physics

To understand how a rainbow works, you first need to understand the physics of light.

A ray of light is made up of many different wavelengths. This includes very short ultraviolet rays, long infrared rays and everything in between (also known as the visible light spectrum). The visible light spectrum ranges from violet waves at 400 nanometers to red waves at 650 nanometers. When all these light rays are grouped together, they form a white mass. However, an object such as a prism can help separate the many rays of light, so that we can see the many colors contained in "white" light.

Prisms help separate light rays through processes called refractions and reflections. These processes change the direction of a light wave: refractions bend the light and reflections bounce it. Only when light strikes a prism at a precise angle (around 42 degrees) can the light rays refract and reflect properly, creating a rainbow.

Conditions required to create a rainbow:

In the laboratory, a rainbow is created using a light bulb and a glass prism, but rainbows can also be created in nature. In an environmental context, the sun serves as the light source for rainbows and water droplets as the prism. When sunlight enters a water droplet, it is refracted and reflected several times and at several angles, until a rainbow is created. These water droplets can be found in a variety of contexts: rain, mist from waterfalls, fog and sea spray all serve as "prisms".

Although we only see rainbows in the form of an arch, rainbows are actually circles (disappearing beyond the horizon). This means that each viewer will see a rainbow differently, depending on where the horizon ends for them. The center of these "circles" is called the anti-sun point - an "imaginary" point opposite the sun that serves as the center of the circle the rainbow surrounds. The size of the rainbow circle is determined by the degree of refraction of the water droplet it penetrates. For example, salt water is highly refractive and will produce a smaller rainbow, while fresh water is less refractive and will produce a larger rainbow.

Rainbow variations

Double rainbow

A double rainbow is a type of rainbow that occurs when two parallel rainbows appear. A double rainbow can occur when sunlight is reflected twice in the same raindrop. As the light is reflected several times, one of the rainbows is usually much fainter and has its colors reversed (red on the inside and violet on the outside, rather than the other way round).

Lunar rainbow

The lunar rainbow is a phenomenon created by light reflected from the moon. The moon itself does not emit light, but reflects the light emitted by the sun, stars and earth. However, the moon's light is around 400,000 times weaker than the sun's, which means that rainbows are often very faint.


Monochrome rainbows

Monochrome rainbows, or red rainbows, are a particular type of rainbow that appears in just one color. These rainbows appear when light has to travel a long distance before reaching water droplets. This long journey means that a large proportion of the shorter blue and violet rays are dispersed, leaving only the red rays. These rainbows usually occur at sunrise or sunset, when the sun is low on the horizon.

Brocken's spectrum

The Brocken Spectrum is a captivating meteorological phenomenon that occurs when the sun shines behind you and casts your shadow on the clouds or fog ahead. Its name comes from the German mountain Brocken, where it was first observed.

When sunlight passes through water droplets or ice crystals in clouds or fog, it breaks up into different colors, creating a rainbow-like separation of hues. This color division is called light scattering. Brocken's Spectrum occurs when these dispersed colors meet your shadow cast on the clouds or fog.

Why is this shadow surrounded by a colored circle in the clouds? Light refraction is the reason. Water droplets or ice crystals deflect sunlight from its path, forming a coloured circle around your shadow.

For optimum observation, high vantage points, such as a mountain top, are recommended. This allows your shadow to extend over a wider area of clouds or fog.

In short, the Brocken Spectrum results from the scattering and refraction of sunlight through cloud or fog particles. It creates a colored circle around your projected shadow, offering a natural and striking spectacle in the sky.

Fun facts

The rainbow is a popular motif in mythology. In many cultures, rainbows are depicted as bridges between the human and supernatural worlds. Examples include the Bifrost of Norse mythology and the rainbow bridges of Japanese and Gabonese cultures. Sometimes, the rainbow is also a symbol of danger, as in certain regions of the Amazon, where it is associated with disease. The most famous myth concerning rainbows is the Irish legend of the leprechaun. In this legend, a leprechaun guards a pot of ice cream at the end of the rainbow, which no one can find.