A person is unable to see objects nearer than 50 cm. He wants to read a book placed at a distance of 25 cm. Find the nature, focal length and power of the lens, he requires for his spectacles.
Object distance, u = 25 cm
The distance of near point, d = 50 cm
Focal length, f = ??
Using the formula: f = 
f = 
= 
= 50 cm
Power = 
= 
= 2D
As focal length is positive therefore he uses double convex lens.
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