NCERT Class X Science Class: Chapter –11. Human Eye and Colourful World – Part-6

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Question 20:

Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colors of the spectrum obtained.

Answer:

Dispersion of light is the phenomenon of splitting of white light into its constituent colors. Dispersion happens when white light passes through a glass prism. The different colors of light waves have different speeds in any medium like glass. They have different bending amounts after refraction. Hence, they show up separately beside each other.

These colors are Violet, indigo, blue, green, yellow, orange, and red. The rainbow also is formed due to dispersion of light waves at tiny water droplet interface with atmospheric air. Please see the diagram for the related geometry.

Image Glass prism, white light beam and spectrum

Image Glass Prism, White Light Beam and Spectrum

Image Glass prism, white light beam and spectrum

Question 21:

Is the position of a star as seen by us its true position? Justify your answer.

Answer:

No. light from stars undergoes atmospheric refraction which occurs in medium of gradually changing refractive index.

Image Apparent star posittion

Image Apparent Star Posittion

Image Apparent star posittion

Question 22:

Why do we see a rainbow in the sky only after rainfall?

Answer:

It is because rainbow is formed by the reflection of sunlight through the suspended rain drops in air after the rain. These tiny rain droplets act as prism and when light passes through these prism they refract into different colors

Image formation of rainbow

Image Formation of Rainbow

Image formation of rainbow

Question 23:

Why is the colour of the clear sky blue?

Answer:

Image colour of the clear sky blue

Image Colour of the Clear Sky Blue

Image colour of the clear sky blue

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering.

Question 24:

What is the difference in colors of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Answer:

During sunrise and sunset the sun appears reddish whereas at noon the sun appears white. Explanation should be given in terms of atmospheric depth travelled by light. Colors are different due to scattering of light by atmospheric particles.

Image sun appears white

Image Sun Appears White

Image sun appears white