NCERT Class X Science Class: Chapter –11. Human Eye and Colourful World – Part-8

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Question 29:

Explain the phenomenon of dispersion of white light through aglass prism, using suitable ray diagram.

Answer:

When the white light such as sunlight is passed through a pin to reduce it to a narrow beam. This narrow beam is made to fall on a triangular glass prism. When a white screen is held on the other side of the prism, we observe a band of seven colors on the screen.

Image White light Spectrum

Image White Light Spectrum

Image White light Spectrum

The color sequence obtained on the screen from its lower end is given by the famous acronym VIBGYOR, where V stands for violet, I stands for Indigo, B stands for Blue, G stands for Green, Y stands for Yellow, O stands for Orange and R stands for Red. The red color bends the least on passing through the prism and violet color bends through maximum angle on passing through the prism.

This is why red is at top and violet is at bottom. Thus, the phenomenon of splitting of white light into its constituent seven color son passing through a glass prism is known as dispersion of light.

Question 30:

How does refraction take place in the atmosphere? Why do starts winkle but not the planets?

Answer:

Our atmosphere consists of various layers of air. as we go up in the atmosphere these layers gradually become rarer when light comes from a distant stars and enter in our atmosphere it refracts from these layers of increasing refractive index after every fraction the light bend towards the normal and to the observer on earth the stars appear not only that due to strong wind and convection currents refractive index of these layers keep on changing these changes the apparent position of the star and the observer on the earth observes that the stars appear twinkling on the other planet do not twinkle because

1. They are not source of light but reflect low intensity light of the sun.

2. They are much closer to the earth and subtend larger angle on the eyes of the observer

Image The Earth's Atmosphere

Image the Earth's Atmosphere

Image The Earth's Atmosphere