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How a rainbow is formed

Image: congerdesign on Pixabay

To understand how a rainbow is formed we need to understand how light acts when it hits water droplets. The drops can be those of rain in the sky, those of a waterfall or those of sprinklers watering a garden.

When the light hits the water, part of it bounces off in all directions and another part passes through it. When the drops are small, the light bounces off making an angle of 42º and decomposing the colors.

The color of the clouds depends on the size of the water droplets. The larger the water droplets, the darker the cloud. Light bounces off water droplets, but the larger they are, the less light they return.

Rainbow at a waterfall. Photo: Fabrizio Frigeni a Unsplash

Small drops of water make the rainbow

Small droplets deflect less light. When they are small enough to deflect the light only 138º you see the colors of the rainbow. The large drops return part of the light the way it came (180º angle) and another part comes out the other way.

As the water droplets get smaller, they bounce off less light and the resulting colors are weaker. There is a moment that you can't see.

The angle of the rainbow

In 1637 René Descartes did the rainbow angle theory and fixed it at 42 degrees. In reality, the arc is seen between 40 and 42 degrees: the red is seen at 42º and the blues at 40º.

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In the morning and evening is when it is most possible for the light returning from the 42º angle to be visible from the ground.

At noon when the sun is highest it also makes an angle of 42º when it meets small water droplets. The difference is that the rainbow is not visible from the ground because the light loses power. The arc will be in the sky (and it will be a circle because there is no land) but it will only be visible from an airplane. And usually only the pilot sees it.

Rainbow at noon
Rainbow with the sun high

The rainbow is individual

When we see a rainbow it is because we are between the sun and the rain at the point where the light returns to 42º from the water droplets.

When the arc appears to us it is because we are located in the middle point of the circle of the arc and the sun is above us.

A cone of light is projected from the drops and what we see is the end. If we go too far we'll never catch him, we won't be able to get under it and see it above. There will be a moment when we lose the angle and it will disappear.

That's why no one ever found the treasure that is hidden at the ends or changed sex for having passed under it, as a Balearic legend says. The men of the past, when they had no explanations, they invented legends about the rainbow.

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Come to think of it, most people have never seen one. You can see it from one town but not from the next town.

If someone were to stand next to us, they would see another arch because their drops are different. If we rose we would see other drops. If we went higher and there was no land we would see that the arc is a big circle.

The rainbow is not visible at noon

It is not common to see a rainbow at noon. However, some have been seen on very clear days with very strong sun. Only in winter can the conditions for a midday rainbow be met.

To get the light to return making an angle of 42º, the sun needs to be low. Only in the morning or evening do we have low sun. When the sun is high the arch is made but no one sees it from the ground. Light loses power along the way.

Rainbow at noon
Rainbow with the sun high

We can make a rainbow disappear

If we put on polarized glasses we will not see the colors of the rainbow. Most sunglasses in sleep.

Being polarized glasses means they block light when polarized. It means that it concentrates on one point. Sunlight is not polarized because it emits in all directions. In contrast, polarized light comes from a point and tends to be very intense and annoying.

In recent years, many people wear them in the summer while leading a normal life outside. All these people will miss the show.

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The rainbow is shaped like an arc but it is a circle

Water drops are round. Since each ray of the sun hits and bounces part of the light, the result is a round one.

More precisely, it is a cone that is projected and we see the end. It is similar to a cinema projector that opens a beam of light from one point but the image appears at a distance.

The most normal is not grasping that the arc is a circle because the earth cuts it.

Full circle of the Arc de Sant Martí
Full circle of the rainbow

At home we can do an experiment to see how a rainbow is formed

It is a bit difficult to understand how a rainbow is formed. And it's harder to see one. But we have an advantage: we can make one at home.

With a hose or some sprinklers we can make the rainbow come out at home and observe how if we move we stop seeing it. In a small version it is easier to see how we stop seeing it when we move a little.

In addition, we can already say that we have seen one!

If you want to see more, we leave you this video:

Rainbow news

Rainbow news

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