Question

A person cannot see objects beyond 40cm.the power of lens to correct vision

Answer 1

Explanation:

A lens which can bring an object at infinity to 40 cm is required here.

So u=∞ and v=−40cm

we know f1=v1−u1

f1=−401−∞1

So f=−40cm=−0.4m

Power of lens P=f1=1/−0.4=−2.5D

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Answer 2

Answer:

2.5D

Explanation:

A lens which can bring an object at infinity to 40 cm is required here.

So u=∞ and v=−40cm

we know  

1/f = 1/-40 - 1/∞

So f=−40cm=−0.4m

Power of lens P = 1/f = 1/-0.4 = -2.5D