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Ray Optics and Optical Instruments

Dispersion By Prism

Suppose on a rainy day when the roads are wet and you are driving a car or riding a bike sometimes you see that the petrol spills on the road. When the petrol mixes with the water we can see different layers of colors on the road. Why does this happen? What is the phenomenon behind this? The answer to this is dispersion. Now let us study about the dispersion of prism.

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Prism

It is a solid figure having two triangular bases and three rectangular surfaces and is the closed surface. The angle between each surface is the angle of the prism. In a prism, the opposite surfaces are equal surfaces and are parallel. There are two refracting surfaces which mean the surface where refraction of light takes place. We are not concerned about other faces.

Dispersion

With the help of a narrow beam of light, a glass prism, and a white wall it is possible to produce the band of seven colors using white light. Keep this arrangement near the window. Place the glass prism in such manner that the sunlight through the window falls on one side of the prism and then on the white wall.

You can see that the light reflected on the wall has several colors. The prism splits the white light into seven different colors. This splitting of white light into many colors is called as a dispersion of light. Dispersion is nothing but splitting of white light into its constituents colors.i.e into seven different colors.

The seven colors are violet, indigo, blue, green, yellow, orange, and red (VIBGYOR). The pattern of color which is obtained is called as a spectrum.  Sometimes in the rainbow, you may not see all the seven colors. This is because of the colors overlap each other.

Let us study about Refraction through a Prism in detail. 

What Causes Dispersion of Prism?

Dispersion of prism takes place because white light entering the prism consists of so many different colors. Each of these different colors has a different wavelength. According to Cauchy’s formula, refractive index (μ) of a material depends upon wavelength  (λ) and is given by,

μ = a+ \( \frac{b}{(λ² )} \) + \( \frac{c}{(λ4)} \), where a, b, c are constants of the material.

Since the wavelength of violet light is smaller than of the red light μv > μr, therefore the violet light has a larger angle than the red light. As a result, the dispersion of white light takes place on the second surface of the prism.

Questions For You

Q1. The dispersion of white light occurs because the angle of ………………….. of different colors is different when passing through the glass prism.

  1. Reflection
  2. Refraction
  3. Incidence
  4. None of these

Answer: B. The dispersion of white light occurs because the angle of refraction of light of different colors is different when passing through the glass prism.

Q2. Which of the following is not a primary color?

  1. Yellow
  2. Red
  3. Green
  4. Blue

Answer: C. The colors which cannot be obtained by mixing of two or more colors are said to be primary colors. Red, yellow and blue are primary colors while green is not.

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