Long Answer-Class 10 -Science- Physics-Chapter 11:The Human Eye And The Colourful World

Class 10 -Science- Long Answer Physics-Chapter 11:The Human Eye And The Colourful World

Q1. With the help of scattering of light, explain the reason for the difference in colours of the Sun as it appears during sunrise/sunset and noon.

Ans :At the time of sunrise or sunset it appears red where as at noon it appear white when it is overhead because at sunrise or sunset blue colour and other shorter wavelength light get scattered away while passing through the atmosphere.

Scattering of longer wavelength is least which reach to the earth. Due to this sun appears red during morning and evening time.

When sun is overhead at noon, sun rays travels smaller distance, due to little scattering of blue and violet colour wavelength almost all wavelengths reach to earth and due to this sun appears white.

 Q2. A person cannot read a book at distances less than 50 cm. Name the defect of vision he is suffering from. How can it be corrected? Draw ray diagrams to show the image formation:

a. by defective eye and

b. after using corrective lens.

Ans : a. Either the hypermetropic eyeball is too short or

b. The ciliary muscle is unable to change the shape of the lens enough to properly focus the image i.e., the focal length of the eye lens is too long. This defect is called hypermetropia.

 Q3. A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.

(b) Why do stars twinkle? Explain.

Ans :  (a) He is suffering from myopia. Causes of myopia: (i) elongation of eye ball. (ii) excessive curvature of cornea. Concave lens of suitable power is used to correct myopia.

(b) Light coming from stars when enter in the Earth’s atmosphere suffers refraction from the atmospheric layers. Since the densities of atmospheric gases changes frequently. Since the stars are point sized and at a far distance so sometimes star appear brighter and sometimes dimmer which gives the impression as the stars twinkling.

Q4. What is meant by scattering of light?

Mention the factor on which it depends. Explain why the colour of the clear sky is blue?

An Astronaut in space finds sky to be dark. Explain reason for this observation.

Ans : (a) Scattering of light is the phenomenon due to which light gets deflected by the atoms, molecules or particles of the medium when light falls on them. These are known as scatterer particles.

(b) Factors on which scattering depends are the size of the atoms, molecules of the scatterer.

(c) When sunlight falls on the atoms or molecules of the gases shorter wavelengths J like of blue colour get scattered more due to which sky appears blue. In space there is no atmosphere, so there is no scattering hence the sky appears dark/black.

Q5. (a) Explain the following terms used in relation to defects of vision and corrections provided for them: (i) Myopia (ii) Astigmatism (iii) Bifocal lenses (iv) Far sightedness. (b) Describe with a ray diagram how a person with myopia can be helped by spectacles.

Ans : (a) (i) Myopia is a kind of defect in human eye due to which a person can see near by objects clearly but unable to see the distant objects. Myopia is due to (i) excessive curvature of cornea (ii) elongation of eye ball.

(ii) Astigmatism is a kind of human eye defect in which he cannot see simultaneously horizontal and vertical lines both. This defect arises either when the cornea or the eye lens or both are not perfectly spherical. It can be corrected by using cylindrical lenses.

(iii) Bifocal lenses with the ageing a person may suffer myopia and hypermetropia. For correction of these defects he is advised to use bifocal lenses whose upper half is concave and lower part is convex to see distant and reading purposes respectively.

(iv) Far sightedness, a person suffering from this defect can see distant objects but unable to see nearby objects clearly. The image of an objects is formed beyond the retina. Such defects can be corrected by using a convex lens. (b) A person with myopic eye may be advice to use a concave lens of suitable power/ focal length.

(b)