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Human Eye and Colourful World – Important Questions
Physics for Class 10
What is myopia? State two reasons due to which this defect is caused.
Myopia or short-nearsightedness: It is a vision defect in which a person can see nearby objects clearly but cannot see the distant objects clearly beyond a certain point. This defect is common among children.
Cause of Myopia: This defect arises due to either of the following two reasons:
(i) The eyeball gets elongated along its axis so that the distance between the eye lens and the retina becomes larger.
(ii) The focal length of the eye lens becomes too short due to the excessive curvature of cornea.
Correction of Myopia: A myopia eye is corrected by using a concave lens of focal length equal to the far point F from the eye. This lens diverges the parallel rays from distant object as if they are coming from the far point F. Finally, the eye lens forms a clear image at the retina.
What will be the colour of sky in the absence of atmosphere? or “The sky appears dark to passengers flying at very high altitudes.” Give reason.
In the absence of any atmosphere, there will be no scattering of sunlight and the sky will appear dark. The sky appears dark to passengers or astronauts flying at high altitudes, as scattering is not prominent at such heights due to thin atmosphere.
What is hypermetropia? What are the two causes of this defect of vision? How can this defect of the eye be corrected?
Hypermetropia or long-sightedness: It is a vision defect in which a person can see the distant object clearly but cannot see the nearby objects clearly.
Cause of hypermetropia: This defect arises due to either of the following two reasons:
(i) The eyeball becomes too small along its axis so that the distance between the eye lens and the retina is reduced.
(ii) The focal length of the eye lens becomes too large resulting in the low converging power of the eye lens.
Correction of hypermetropia: A hypermetropia eye is corrected by using a convex lens of suitable focal length. This lens diverges the rays such that the rays coming from normal near point N appear to come after refraction, from near point N’ of the defected eye. That is a virtual image of the object placed at N is formed at N’. Then the eye lens forms a clear image at the retina.
What is astigmatism? How is it caused? How is it corrected? Or What is astigmatism and how is this vision defect counteracted?
Astigmatism: It is a defect of vision in which a person cannot simultaneously see both the horizontal and vertical views of an object with same clarity.
Cause of astigmatism: This defect occurs when the cornea of the eye is not perfectly spherical. This results in objects in one direction being well focussed while those in perpendicular direction are not well focussed.
Correction of astigmatism: Astigmatism can be corrected by using cylindrical lenses. They have different curvatures in horizontal and vertical directions and so they can be oriented suitably to compensate for the irregularities in the cornea.
More Questions for Exams
Why do we see stars appear twinkling, whereas planets do not twinkle?
A star sometimes appears brighter and some other times fainter. What is this defect called? State reason for this effect.
Twinkling of stars: The apparent position of a star is slightly different from the actual position due to refraction of starlight by the atmosphere. Further, this apparent position is not stationary but keeps on changing due to the change in atmospheric conditions like density, temperature, etc. the path of the rays of light coming from the stars goes on varying slightly. The amount of light entering our eyes from a particular star increases or decreases randomly with time. Sometimes, the star appears bright and other times, it appears fainter. This gives rise to the twinkling effect of the star.
The planets do not show twinkling effect: As the planets are much closer to the earth, the amount of light received from them is much greater and the fluctuations caused in the amount of light due to atmospheric refraction are negligible as compared to the amount of light received from them.
What is presbyopia? Write two causes of this defect.
Presbyopia: this defect is similar to hypermetropia i.e., a person having this defect cannot see nearby objects distinctly, but can see distant objects without any difficulty. This defect differs from hypermetropia in the cause by which it is produced. It usually occurs in elderly persons. Due to the stiffening of the ciliary muscles, the eye lens loses flexibility and hence the accommodating power of the eye lens decreases.
“Stars appear higher than they actually are.” Give reason.
Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The stars appear slightly higher (above) than their actual position when viewed near the horizon.
Why is the colour of the clear sky blue? Explain.
The blue colour of the sky is due to the scattering of the sunlight by the molecules of the atmosphere. The molecules of air, such as N2 and O2, have sizes smaller than the wavelength of visible light. As sunlight passes through the atmosphere, these molecules absorb some amount of sunlight and re-emit it. They scatter blue light of shorter wavelength more strongly than red light of longer wavelength. When we look at the sky, the scattered light enters our eyes and this light contains blue light in a larger proportion. This is why, the sky appears blue.