10. The near point of a man suffering from hypermetropia is 150 cm. If he wants to read a book kept at a distance of 25 cm, what kind of lens is necessary ? Also, find the focal length of the lens?
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The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).
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The person needs a convex lens to rectify this defect.
Calculation of power of the lens:
This hypermetropic eye can see the nearby object kept at 25 cm clearly if the image is formed at its own near point i.e. 50 cm.
Object distance, u=−25 cm
Image distance, v=−50 cm
v
1
−
u
1
=
f
1
−50
1
−
−25
1
=
f
1
f=+50 cm
P=
f(in cm)
100
=
50
100
=+2D