Question

(a) Name two defects of vision. Mention two main causes of each defect.


(b) A person uses spectacles having lenses of focal length – 2.0 m.
Determine the nature and power of the lenses used in his
spectacles.



class 10th science
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Answer 1

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• Sometimes the eye of a person cannot focus on the image of an object on the retina properly, resulting in defects of vision and they are: Myopia (short-sightedness or near-sightedness) Myopia is caused because of: Due to high converging power of eye lens.
• As power is , therefore focal length is which is so, focal length is positive which is for convex lens and a convex lens is used for correction of hypermetropia. So, the person is suffering from hypermetropia, hence correct option is .

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Answer 2

Answer:

a) Two defects of vision

i) Myopia

This is the defect of eyes in which the person has difficulty in seeing distant objects but can easily see nearby objects. This eye defect is also called as nearsightedness.

For the persons with Myopia, the far point is nearer than infinity. The image of the object placed at a distance is formed in front of the retina and not on the retina.

It is caused by:

1) Excessive curvature of the eye lens

2) Elongation of the eyeball

ii) Hypermetropia

A person can see distant objects distinctly but cannot see nearby objects so clearly in this case, the image is formed behind the retina. This defect of the eye is hypermetropia.

Causes of hypermetropia: This defect arises because either

(i) the focal length of the eye lens is too long, or

(ii) the eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.

b) The person is using Convex lens

and of power 1/f = 1/2 = 0.5D