A person is unable to see objects distinctly placed within 50cm from his eyes. Mention the type of lens used by him for the defect and calculate its power.
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Hypermetropia
Convex lens
1/
Convex lens
1/
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The person should be using convex lens( facing outwards) as he is suffering from hypermetropia or farsightedness.
Power of the lens is +2 D.
Explanation:
Power = 1/f ( in m) , where f is the focal length of the lens in meters.
Also we know, 1/f = 1/v - 1/u ( lens formula), where u is the object distance and v is the image distance
We take the near point of a normal eye as 25 cm and so object distance becomes -25 cm and image distance -50cm.
From the given formula, we calculate the f as +50 cm or 0.5 m
So, the power of the lens is = 1/0.5 = 2 dioptre(D).
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