Question

A person is suffering from myopia, his far distance is 5 m. What is the

focal length of his eye lens​

Answer 1

Answer:

Given :

Distance of far point:X=5cm.

focal length of his eye lens to view distant objects correctly:f= -x = - 5m

Ans: power of the lens:P = 1/f

= 1/(-5)

= - 0.5 d

The focal length of his eye lens is -0.5

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Answer 2

Given:

A person is suffering from myopia, his far distance is 5 m.

To find:

Focal length of lens he needs?

Calculation:

In myopia, the far point comes from infinity to somewhere forward (here 5 m).

So, object from infinity has to form an virtual object at 5 m using a corrective eye lens. [That eye lens have to be CONCAVE LENS]

$\rm \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$\rm \implies\dfrac{1}{f} = \dfrac{1}{( - 5)} - \dfrac{1}{ \infty}$

$\rm \implies\dfrac{1}{f} = - \dfrac{1}{ 5} - 0$

$\rm \implies\dfrac{1}{f} = - \dfrac{1}{ 5}$

$\rm \implies f = -5m$

So, power will be :

$\rm \: P = \dfrac{1}{f \: (in \: metres)}$

$\rm \implies \: P = \dfrac{1}{ - 5}$

$\rm \implies \: P = - 0.2 \: D$

So, power of lens is -0.2 Dioptre.