Q. 7 B 3.9( 58 Votes )

The near point of a hypermetropic eye is 1m. What is the nature and power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Answer :

The eye defect called Hypermetropia is corrected by using convex lens spectacles.

We will first calculate the focal length of the convex lens required in this case. For hypermetropic eye can see the nearby object kept at 25 cm (at the near point if normal eye) clearly if the image of this object is formed at its own near the point which is 1 meter here. So, in this case :

Object distance, u = -25 cm (Normal near point)

Image distance, v = -1 m (Near point of this defective eye) = -100 cm

Focal length, f=? (To be calculated)

Putting these values in the lens formula,

By putting in values

We get : 


Or, 



Power of Lens needs to be calculated,
Given by P=
∴ We convert 33.3 cm into meter i.e 0.33m
P=
= +3.0 D
Thus, the power of the convex lens required is +3.0 diopters.

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