Q. 213.6( 7 Votes )

# The far point of

The defect called myopia is corrected by using a concave lens. We will now calculate the focal length of the concave lens required in this case. The far point of the myopic person is 80 cm. This means that this person can see the distant object (kept at infinity) clearly if the image of this distant object is formed at his far point.

Object distance, u = ∞ (infinity)

Image distance, v = -80 cm (Far point, in front of lens)

And, Focal length, f=? (To be calculated)

Putting these values in the lens formula:

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

We get

$\frac{1}{-80}-\frac{1}{\infty}=\frac{1}{f}$

or $\frac{1}{f}=\frac{1}{-80}$

or f = -80 cm

Thus, the focal length of the required concave lens is 80 cm. We will now calculate its power. Please note that the focal length, f= -80 cm = -0.8 m.

$Power, P=\frac{1}{f(meter)}$

$Power, P=\frac{1}{-0.8} = -1.25 D$

So, the power of concave lens required is  -1.25D

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