You must have seen and appreciated the spectacular colors of the rainbow. Have you ever thought about how the white light of the sun gives us the various colors of the rainbow? To understand this, we shall first understand the dispersion of light and refraction through a prism.

 

What is a dispersion of light?

 

When a narrow beam of sunlight passing through a pinhole is made to fall on a triangular glass prism, we see a band of seven colors on the screen on the other side. This phenomenon of splitting of white light into its constituent seven colors on passing through a glass prism is called dispersion of light and the band of colors is known as spectrum.

 

The color sequence obtained on the screen from its lower end is given by the famous acronym �VIBGYOR� in which V stands for the color violet, I for indigo, B for blue, G for green, Y for yellow, O for orange, and�R stands for red.

 

Aren�t you wondering how we get these colors? Why these colors are arranged in this particular sequence?

 

The answer to these questions is simple. Sunlight or white light is a mixture of seven other colors namely violet, indigo, blue, green, yellow, orange, and red. Each of these colors has its own characteristic wavelength and frequency. Though all these colors of light travel in vacuum or air with the same speed, their speeds in any other refracting medium are different.

 

For example: In the glass, violet color has the minimum speed while red color has the maximum speed.
Due to the difference in their speeds, the different colors of light bend through different angles with respect to the incident ray.

 

This is the reason why the different colors emerge along different paths and become distinct, thus giving a band of distinct colors of the spectrum.

 

Isaac Newton was the first to use a glass prism for obtaining the spectrum of sunlight. He further tried to split these colors using a second prism but failed.

 

However, when he placed a second identical prism in an inverted position with respect to the first prism so as to allow the colors to pass through it, he observed a beam of white light emerging from the other side of the second prism. This observation made Newton conclude that sunlight is made up of seven colors.

 

A rainbow is a natural spectrum appearing in the sky after a rain shower.

 

It is caused by the dispersion of light (sunlight) by tiny water droplets present in the atmosphere. The water droplets act as small prisms. They refract and disperse the incident sunlight, and then reflect it internally before finally refracting it again on coming out of the raindrop.

 

In this way, different colors reach the observer�s eye. It should also be observed that a rainbow is always formed in the direction opposite to that of the Sun.

 

Refraction of light through a prism

 

You have learnt how light gets refracted through a rectangular glassslab. For parallel refracting surfaces, as in a glass slab, the emergent rayis parallel to the incident ray. However, it is slightly displaced laterally. How would light get refracted through a transparent prism?

 

Consider a triangular glass prism. It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism.

 

Let us now do an activity to study the refraction of light through a triangular glass prism.

 

1. Fix a sheet of white paper on a drawing board using drawing pins.
2. Place a glass prism on it in such a way that it rests on its triangular base. Trace the outline of the prism using a pencil.
3. Draw a straight line PE inclined to one of the refracting surfaces, say AB, of the prism.
4. Fix two pins, say at points P and Q, on the line PE as shown in Fig. 11.4.
5. Look for the images of the pins, fixed at P and Q, through the other face AC.
6. Fix two more pins, at points R and S, such that the pins at R and S and the images of the pins at P and Q lie on the same straight line.
7. Remove the pins and the glass prism.
8. The line PE meets the boundary of the prism at point E (see Fig. 11.4). Similarly, join and produce the points R and S. Let these lines meet the boundary of the prism at E and F, respectively.
9. Join E and F.
10. Draw perpendiculars to the refracting surfaces AB and AC of the prism at points E and F, respectively.
11. Mark the angle of incidence (?i), the angle of refraction (?r), and the angle of emergence (?e) as shown

 

Here PE is the incident ray, EF is the refracted ray and FS is the emergent ray. You may note that a ray of light is entering from air to glass at the first surface AB. The light rayon refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. Compare the angle of incidence and the angle of refraction at each refracting surface of the prism.

 

Is this similar to the kind of bending that occurs in a glass slab? The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case, ?D is the angle of deviation.

 

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