A far-sighted person cannot focus distinctly objects closer than 120 cm. The lens that will permit him to read from a distance of 40 cm will have a focal length:

+ 30 cm

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- 30 cm

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+ 60 cm

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- 60 cm

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Solution

The correct option is C + 60 cm
Given,

Near point, $v = - 120 c m$

Reading Distance, $u = - 40 c m$

From lens formula,

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 120} - \frac{1}{- 40} = \frac{1}{60}$

$f = + 60 c m$

Focal length of lens $f = + 60 c m$

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A far-sighted person cannot focus distinctly objects closer than 120 cm. The lens that will permit him to read from a distance of 40 cm will have a focal length:

The correct option is C + 60 cmGiven, Near point, v=−120cm Reading Distance, u=−40cm From lens formula, 1f=1v−1u=1−120−1−40=160 f=+60cm Focal length of lens f=+60cm

The correct option is C + 60 cm
Given,

Near point, $v = - 120 c m$

Reading Distance, $u = - 40 c m$

From lens formula,

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 120} - \frac{1}{- 40} = \frac{1}{60}$

$f = + 60 c m$

Focal length of lens $f = + 60 c m$