Question

Name the four common defects of vision and state the cause of each defect.
A person cannot see distinctly objects placed beyond 2 metres.
State the nature and focal length of the lens which could be used to correct this defect.​

Answer 1

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The four common defects of vision and their causes are as follows:-

1.Myopia:

It is caused when the eye ball becomes elongated or when the lens has excessive curvature.

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2.Hypermetropia:

It is caused when the eye ball becomes small or when the lens has decreased curvature.

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3.Presbyopia

It is caused in old age when the power of accommodation decreases.

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4.Colour Blindness

It is caused when rod cells decrease in number or capacity.

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✦ A person suffering from myopia

So, a concave lens is needed whose focal length f = - 2 m

⇒P = 1/f = - 0.5 D

Answer 2

Explanation:

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