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With new discoveries and innovations constantly being made, the study of Physics Topics remains a vibrant and exciting field of research.
What is Dispersion and Scattering of Light ?
In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colours. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen (see Figure).
The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is called spectrum of white light. The seven colours of the spectrum are : Red, Orange, Yellow, Green, Blue, Indigo and Violet (see Figure).
In Figure, a beam of white light AB is passed into a glass prism PQR. This beam of white light splits on entering the glass prism and forms a broad patch of seven colours (spectrum) on a white screen S, placed on the other side of the prism (see Figure).
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The formation of spectrum of seven colours shows that white light is a mixture of seven colours. White light can he sunlight. So, we can now say that the sunlight consists of seven colours, The role of glass prism is only to separate the seven colours of white light.
From the above discussion we conclude that white light can be split into seven colours by using a prism. The colours of which the white light (such as sunlight) is made are : red, orange, yellow, green, blue, indigo and violet.
Activity 15
We can perform an activity to show that white light consists of seven colours, as follows : Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the glass prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall (which will act as screen).
A patch of seven colours will be formed on the sheet of white paper (or on white wall). This shows that sunlight consists of seven colours. Please note that bulb light is also white light. So, bulb light coming through a narrow slit can also be used to perform this activity.
The Rainbow
One of the most beautiful examples of spectrum formed by the dispersion (or splitting) of sunlight is provided by nature in the form of a rainbow. The rainbow is an arch of seven colours seen in the sky. It is called ‘Indradhanush’ in Hindi. The ‘seven coloured’ rainbow is formed in the sky just after rain when the sun is shining (see Figure).
We can see a rainbow in the sky only when our back is towards the sun. The rainbow is produced by the dispersion of sunlight by tiny rain-drops suspended in the atmosphere. This happens as follows : Just after rain, there are a large number of small water drops suspended in the atmosphere.
Each of these water drops acts like a small prism. As the white sunlight enters and leaves these tiny water drops, the various coloured rays of sunlight are deviated by different amounts and a long arch of seven colours called ‘rainbow’ is formed in the sky.
The seven colours present in a rainbow are : red, orange, yellow, green, blue, indigo, and violet. It is usually not easy to distinguish all the seven colours of white light in a rainbow because many colours overlap over one another. The formation of rainbow having seven colours shows that white sunlight consists of seven colours. Thus, a rainbow is a spectrum of sunlight.
Many times we have seen that when we blow soap bubbles, they appear colourful (having many different colours). The formation of colourful soap bubbles tells us that the white sunlight is a mixture of many different colours. Similarly, when white sunlight is reflected from the surface of a compact disc (CD), we see many colours (see Figure). This observation also tells us that white sunlight is a mixture of many colours.
Mixing Seven Coloured Lights to Make White Light
We have just studied that white light consists of seven colours. An important question now arises : Can we mix these seven colours to get white light ? The answer is : Yes, we can mix seven coloured lights to get back white light. We will now describe an activity which will demonstrate that we can make white light by mixing seven coloured lights. This can be done by using Newton’s disc as follows.
Activity 16
Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven equal segments. Paint the seven colours of white light (red, orange, yellow, green, blue, indigo and violet) on these segments [as shown in Figure (a)]. Instead of painting the segments, we can also paste seven coloured papers on these segments.
This disc painted with the seven colours of spectrum is called Newton’s disc. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Make sure that the disc can rotate freely.
Now, rotate the disc quickly in daylight. When the disc is rotated very fast, all the seven colours mix together due to which the disc appears to be white [see Figure (b)]. Thus, the rapid rotation of Newton’s colour disc tells us that mixing of seven colours of the spectrum can give us white light.
We can also make a ‘top’ or ‘spinning top’ (lattu) by fixing a small circular disc painted with the seven colours of spectrum on a small pencil [as shown in Figure (a)]. Let us make this top ‘spin’. As the top spins, its disc looks white because the seven colours of spectrum merge to make white light [see Figure (b)].
Sir Isaac Newton had observed for the first time that white ray such as sunlight is a mixture of different colours. He had observed that when a ray of white light is refracted through a prism it is separated into seven colours forming a band.
Experiment: It can be shown by an experiment that white light consists of seven colours i.e., it is polychromatic.
A ray of white light passing through a narrow slit S is incident on a refracting face of a glass prism P. On passing through the prism the constituent colours are separated and emerge as a band of seven colours on the white screen placed on the other side of the prism [Fig.]. This band of colours on the screen is called spectrum.
From bottom upwards, the colours of the band are violet, indigo, blue, green, yellow, orange, red. This band of seven colours is together commonly called VIBGYOR, the word being formed by the initial letters of the colours in the spectrum arranged in the order in which they have been written.
Definition: The phenomenon of splitting up of polychro-matic light into different colours is called dispersion of light.
If we observe the spectrum, we see that the deviation of different rays are different. The deviation of violet ray is maximum and that of red ray is minimum. For this, it is said that the refractivity of light for different colours are different. The magnitude of deviation of yellow ray is almost an average of those for red and violet rays. So yellow light is called the mean ray. Again deviation also depends on the refractive index of the material of the prism.
The more the refractive index of the prism, the more is the devi-ation. The refractive index of a medium is minimum for red light and maximum for violet light.
The order of refractive indices of a medium for red, orange, yellow, green, blue, indigo and violet light is,
μr < μ0 < μy < μg < μb < μi < μv
Cause of dispersion Of light: In vacuum or in air, all the different coloured rays travel with the same speed. But through any other medium they travel with different speeds. According to wave theory of light refractive index of a medium is given by the ratio of speed of light in vacuum to the speed of light in that medium. Velocity of red ray, through glass, is greater than violet ray.
So a medium has different refractive indices for different colours. Refractive index of red ray for a medium, is less than other rays for the same medium. We know that, if refractive index of a medium decreases then deviation of ray also decreases. That’s why violet ray is deviated most and red ray is least deviated.
So, in short it can be said that dispersion of polychromatic light takes place due to difference in the velocities of the components of the light in a medium. The medium in which dispersion of light takes place is called a dispersive medium. Dispersion of white light takes place in glass. So glass is a dispersive medium. But vacuum or air is not a dispersive medium.
Experiments Related to Dispersion of Light
A prism does not create colour; it just splrts white light into different colours; Through the narrow slit S, white light is incident on the prism P1 and after dispersion it creates a VR spectrum on screen C1 [Fig.], There is a narrow slit S1 on C1. Displacing the screen up and down, a particular light (let yellow light) of the spectrum is sent through the slit and incident on prism P2. The ray emerging from second prism is incident on a screen C2. It is observed that the ray is deviated towards base, it means that the ray has suffered deviation but the ray is not splitting into different colours i.e, no spectrum is visible.
Same incident happens for other colours. So if colour has been created by prism then spectrum would be observed after refrac-tion from second prism. From this experiment it is further proved that the colours present in white light are pure, hence no dispersion is possible for these colours.
As we get seven different colours by the dispersion of white light, so we can get white light by the recombination of the seven colours, because white light is just a mixture of the different con-stituents.
By two similar prisms: Two exactly similar prisms of same material P1 and P2 are placed side by side [Fig.], Similar refracting surfaces of the two prisms are parallel (A1B1 || A2B2 and A1C1 || A2C2) and their bases are on the opposite sides. Through the narrow slit S white light is incident on the prism P1 and after dispersion it forms a spectrum.
The different colours of the spectrum after passing through the prism P2 recombine and the emergent beam received on a screen appears white.
By Newton’s colour disc: Newton’s colour disc [Fig.] is a circular card-board disc. It is divided usually into four quadrants. Each quadrant is painted with the vibgyor colours in the proportion in which they are present in white light. If the disc is now rotated quickly it appears white. Visual impression per-sists for about \(\frac{1}{10}\)th of a second even after the stimulus is removed. The phenomenon is called persistence of vision. So when the disc is rotated quickly the disc appears white.