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All you need to know about rainbows

Have you ever contemplate a rainbow after the rain?

To better understand this physics phenomenon first thing we need to know is that sunlight is a mixture of seven colours and together they form a white colour light.

Whenever it rains, droplets of water stay in the air and when the sunlight passes through these droplets, it is divided into seven colours: red, orange, yellow, green, blue, indigo and violet.

This phenomenon is called the dispersion, or composition of white light.

In 1672 the English physicist Isaac Newton studied and elaborated theories on the mechanism of light scattering. Despite his corpuscular interpretation of light, Newton was able to explain that white light was actually composed of all other colours and that these colours were relative to the frequencies (or wavelengths) of the light.

When white light, whether it comes from the sun or from an incandescent lamp, changes from one propagation medium to another it undergoes refraction, that is, there is a change in the speed of propagation.

This change in the means of propagation causes white light to decompose into infinite rays of monochromatic lights, known as the seven colours of the rainbow. This physical process, therefore, constitutes the decomposition of white light.

Although it is known that white light is composed of a multitude of colours, such light colours do not have the same behaviour when they change from one medium of propagation to another. The light that comes closest to normal is violet; next are the colours: indigo, blue, green, yellow, orange and red. The colours that make up white light are called the light spectrum.

In the image below we can see the decomposition of white light in a prism. In the prism, the decomposition of white light is more accentuated by the fact that it undergoes two refractions, that is, it undergoes refraction on the first face and later on the second face.

Optical glass triangular prism. Image by Dobromir Hristov at Pexels.

Observe the spectrum that results from the passage of white light through a prism.

Unfortunately, the rainbow is not visible for a long time as it falls apart if the sun changes position or if a strong wind spreads the droplets of water.

To better understand the phenomenon, here is a video in which Pedro Stuginski – a mechanical engineer with physics knowledge – talks about rainbows.


Would you like to know more about rainbows, light and prisms? Leave your questions and suggestions in the comments and we will answer them for you.

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