What is a rainbow?

We explain what a rainbow is and the types of rainbows that exist. How it forms, why it has seven colors and places with more rainbows.

1. What is a rainbow?

The rainbow is a meteorological phenomenon and is presented in the form of a multicolored arc of light , by the refraction of sunlight (white light) in the raindrops that are suspended in the atmosphere . In its most intense manifestation you can see the seven colors : red at the top or outside and successively orange, yellow, green, cyan (or turquoise), blue and violet at the bottom or inside.

2. Rainbow types

The types of rainbows depend on different atmospheric conditions that influence the projection mode of the light arc, and the main ones include:

• Primary Rainbow It is the best known and usually appears after a storm or in areas of water slope that splash, such as a waterfall.
• Secondary rainbow Also called “double rainbow”, it is formed above the primary rainbow, with inverted colors.
• Supernumerary rainbow. It is rare to see and consists in the projection of several faint rainbows simultaneously, product of the diffraction of sunlight.
• Red rainbow. Also known as “monochrome rainbow”, it is formed after rain and during sunrise or sunset, with the sun located very low or near the horizon.
• Circhorizontal arch. Also known as “rainbow of fire”, it appears very rarely and is formed from small drops of water in cirrus clouds, where the spectrum of colors is projected (instead of appearing in the form of an arc).
• Twin Rainbow It is very rare to see and is projected with two arches that, unlike the secondary rainbow, originate from a single base point and their colors are not reversed.

However, for the scientific environment the classification of rainbows is even more thorough . A study published in 2015 by the National Meteorological Research Center of France, headed by researcher Jean Ricard, establishes that there are twelve different types of rainbows and that this classification includes the amount of visible colors, the projection of multiple arcs and the alteration of the Sky color between each arch. All these parameters establish small differences between the types of rainbows described above.

3. How does a rainbow form?

The rainbow is formed by the decomposition of a ray of light that passes through a drop of water suspended in the atmosphere. When a ray of light crosses a surface that divides two spaces of different densities (in this case, the atmosphere air and the water drop), the ray refracts its path, that is, the path angle is bent or slightly changed . It is then reflected (bounces) on one of the faces inside the drop and, when leaving, the ray of light is refracted again.

The rainbow can appear in any place where there is humidity in the air , for example, near the splashes of a river or by the dew of the sea, and with the sun located in a specific position: less than 42º above the horizon. The observer must be located facing the raindrops and with the Sun behind him.

4. Why does the rainbow have seven colors?

The ray of light that crosses the drop of water generates a decomposition of white light in different wavelengths. These lengths are different from each other and give rise to the different colors of the rainbow. This concept of the “decomposition of light” was demonstrated by Isaac Newton in the seventeenth century , through an experiment with a glass prism that was pierced by a ray of sunlight. He verified that the white light was formed by bands of colors that could be separated and displayed individually (as is the case with the rainbow).

5. Places with more rainbows

Among the places where rainbows are most visible, the following stand out:

• Machu Pichu. Located in the mountain range of Los Andes, in southern Peru, it is an ancient Inca town of the fifteenth century.
• Victoria Falls. Located on the border between Zimbabwe and Zambia, in Africa.
• Iguazu Falls. Located on the border between the countries Argentina and Brazil.
• Masai Mara Nature Reserve. Located in Kenya, Africa.
• Montes Tatras National Park. Located in the Tatra Mountains, Slovakia.
• Jasper National Park Located in the Rocky Mountains, in Canada (declared a World Heritage Site by UNESCO in 1984).
• Norfolk Located in England, it is a city with fine sandy beaches.
• Grinell Point Mountain. Located in the Glacier National Park, in Montana, United States.
• National Park of the Valley of the Flowers. Located in the state of Uttaranchal, in India (declared a World Heritage Site by UNESCO in 1998)
• Mount Fuji. Located west of Tokyo, Japan. It is the highest peak on the island of Honshu, located west of Tokyo, in Japan.