How are Rainbows Formed

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How are Rainbows Formed

A rainbow is a colorful bow appearing in the sky when it suddenly rains on a bright sunny day.

What is a Rainbow?

Sunlight is composed of light of varying wavelengths. Whilst the shorter wavelength light appears as blue, violet and indigo in color, the longer wavelength light appears as red, orange and yellow in color. When this sunlight passes through a raindrop in the air, it takes a complicated path. Initially light splits into the ‘VIBGYOR’ colors. This light then reflects off the back surface of the raindrop and then goes back to the other side in which the light first passed, forming an arch. The light thus emerging from several of the raindrops create a rainbow.

It is not always that every time it rains we can witness this colorful rainbow in the sky. The prerequisite for rainbow formation is a brightly shining sun from one side and rain coming from the other direction.

When you look at a rainbow, you would see that the sun is behind you. There is a fixed critical angle where the light from the sun is reflected back to you. If there is a water droplet in the right place and at the time of a rain, there will always be raindrops in the right place because drops falling behind will replace the earlier ones. A rainbow with the brightest colors is seen when the sun is quite low on a clear sky just behind us, and a heavy rain just in front of us.

Why should the rainbow be curved? Why can’t it be a straight line across the two horizons?

A rainbow is always curved just for the reason that all the angles of the water droplet have to be just right for it to reflect some light back to you, standing on the ground. So when there is sunlight behind a viewer, only those water droplets that have the same angle formed by the viewer, the droplet, and the sun (which is approximately 42 degrees) will produce the rainbow effect. Other droplets send their light somewhere else, and if the person moves to a different location, a few more droplets are needed to form another rainbow that the viewer witnesses in the new place.

This is also the reason why it is said that we cannot reach the end of a rainbow. Wherever a person stands, it is only the faraway droplets of water that reflect and bend sunlight to result in a rainbow.

A rainbow may look like a half circle when it is formed at sunrise or sunset because the light at that time travels almost horizontally towards the earth. However, as the sun rises high in the sky, the shape of the rainbow would be like an arch rather than a half circle.

Interestingly, from an airplane a rainbow is seen as a full circle! And how do you like this?

The ‘VIBGYOR’ colors of a rainbow are due to the fact that each rain droplet behaves like an independent prism which splits sunlight into its constituent colors.

A simple demo to form a rainbow under normal conditions!: Take a spray gun and fill it with water. Go out on a sunny day and – with the sun behind you – spray water in front of you along all directions to see rainbows popping all around.

Rainbow Image Gallery