Question

the far point of a myopic person is 40 cm what is the nature and power of lens required to correct the defect

Answer 1

u =(infinity), v = - 40 cm, f = ?
Using the lens formula, we have
= 1/v-1/u =1/-40- 1/infinity = 1/-40 = - 40 cm= -0.4 m
Hence, power of lens to be used is
P= 1/f = 1/-0.4 = -2.5D

Answer 2

The nature and power of lens required to correct the defect is -2.5D .

Explanation:

     As given in the question we know that the “v” in this case would be - 40 cm while the “u” that we are talking about would be infinity now we have to put these values in the lens formula and have to find focal length.

Using the lens formula,  and applying the values we will get

                             $\frac{1}{v}-\frac{1}{u} = \frac{1}{-40} - \frac{1}{∞}$

Since 1 by infinity would be zero we will get

                                     $\frac{1}{f} = \frac{1}{-40}$

f= - 40 cm= -0.4 m

So, to correct this defect the power of lens that has to be used would be

P= 1/f

= 1/0.4

= -2.5 D

The nature and power of lens required to correct the defect is -2.5D .

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