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Rainbows are the delight of the monsoons. Learn about how they are formed and the numerous variations.

There are bridges on the rivers,
As pretty as you please;
But the bow that bridges heaven,
And overtops the trees,
And builds a road from earth to sky,
Is prettier far than these.

Thus goes a verse from the poem 'The Rainbow' by well-loved children's poet Christina Rossetti. Numerous yarns have been spun; various landscapes painted with the rainbow serving as inspiration. Ever wondered what exactly are rainbows? How in the world do all the colours sweep across the sky?

What Are Rainbows?

Sunlight appears plain white in colour. However, appearances can be deceptive. The plain white light is actually a combination of numerous colourful shades ranging from violet to red. Yes, the rainbow is not a colourful strip painted across the sky. It is our own ordinary sunlight transformed by water vapour especially during monsoons. Water vapour helps to split white light into the multiple colours that are hidden inside.

The transformation of white light into a rainbow can be explained by the science called Optics (a branch of physical science that deals with light and vision) and meteorology (the science of atmosphere and weather). When a beam of white light is shone through water, it bends as water impedes its speed. This phenomenon is called refraction. Each constituent colour of the white light travels at a different speed. Red travels faster and is least refracted, violet is most refracted. The result is that light gets split into its constituent colour spectrum. This is called dispersion. A rainbow is nothing but white light dispersed in the sky. It is formed when the sun shines on droplets of moisture in the atmosphere. That is why it is seen during the rains. If you observe closely, red constitutes the outer part of the arch while violet constitutes the inner part.

Though you may have memorised the VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red), in reality, a rainbow spans a continuous spectrum of colours. It is not just 7 colours; there are hundreds of other shades in between. Rainbows are generally bow-shaped but stripes, circles or flames may also be spotted at times.

How Are They Formed?

This property of sunlight was first demonstrated by Sir Isaac Newton in 1666. Light of different colours is refracted by different amounts in various media. Before the dispersion, sunlight is refracted thrice—first as it enters the raindrop, then from the back of the raindrop, and finally as the light leaves its surface. Refraction is nothing but the change of direction of a ray of light as it passes from one medium to another with a different wave velocity. This refraction happens over a wide range of angles. The angle depends on the size of the raindrop and its refractive index. The refractive index of a medium is a measure for how much the speed of light is reduced inside the medium. Have you noticed that rainbows near the sea are smaller than rainbows from the rain? This is because sea has a higher refractive index than rain.

The rainbow is actually an optical illusion. Both, the position of the observer stands as well as the position of the Sun determine the location of the rainbow. This stems from the fact that all raindrops reflect and refract the sunlight but only some of them reach your eyes as a rainbow. From where you stand, rainbows will always appear in a direction opposite to the Sun's. Why is it that you do not spot a rainbow after every shower of rain? The bow must appear at an angle of 40o—42o to the line between the observer's head and its shadow. If the sun is higher than 42o, there are not enough raindrops between the horizon (eye height) and the ground to form a rainbow. The rainbow then goes below the horizon and is thus not seen.

Types of Rainbows

There can be more than one rainbow at a time. A secondary rainbow is a dimmer and thicker rainbow seen outside the primary rainbow. It occurs when there is a double reflection of sunlight inside the raindrops. It is at an angle of 50o—53o. The colours of a secondary rainbow are inverted with blue on the outside and red on the inside. The portion of sky between the primary and secondary rainbows remains unlit. It is called the Alexander's Band.

In rare case, a triple rainbow—with three arcs—may be seen. Observers have even reported seeing quadruple rainbows. In this, there is a dim outermost arc with a rippling and pulsating appearance. The reason why you seldom see them is that they appear on the same side of the sky as the sun. A supernumerary rainbow consists of several faint rainbows on the inner side of the primary rainbow. Sometimes these may also occur outside the secondary rainbow. These additional rainbows are detached from the original and have pastel hues. Unlike normal rainbows, they are caused due to interference between different rays of light with varying paths and lengths.

A "moonbow", also called lunar rainbow or night-time rainbow, may be seen on brilliantly moonlit nights. As we cannot see very well in dim light, the rainbow is perceived to be white in colour.

Make Your Own Rainbow

You can try making your own rainbow indoors.

  • Place a plain glass bowl of water on a desk or table near a blank wall.

  • Put a small flat mirror in the water such that the mirror rests at a 45-degree angle against the side of the bowl. You may use a protractor to determine the angle.

  • Now stand behind the mirror and shine a flashlight straight down on the mirror. You may need to darken the room.

  • You will see a rainbow on the wall opposite the mirror. This is because the light from the flashlight is broken into its component colours by the water and reflected back by the mirror.
The most breathtaking rainbow is seen when half of the sky is still dark with draining clouds and you are at a spot with clear sky overhead. It is indeed marvellous that mere sunlight can create such a beautiful phenomenon. Rainbows prove that science can be as exciting as art. Rainbows are beautiful, but they become even more beautiful when you understand the science behind them.

Do you enjoy watching rainbows? Can you recite the VIBGYOR correctly? Was it interesting to know about the formation of rainbows? Did you try making your own rainbow? To tell us about your views and experiences, click here.

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