Question

A nearsighted person cannot see objects beyond 80 cm from his eyes. Which one of the followingcombinations represents the correct focal length and the refractive power of the contact lenses thatwill enable him to see the distant objects clearly?A) -80 cm, -1.3 dioptersB) -1.3 cm, +1.3 dioptersC) -80 cm, +1.3 dioptersD) +80 cm, +1.3 dioptersE) +80 cm, -1.3 diopters

Answer 1

Option (A) is correct.

Step-by-step explanation:

for a near sighted person, distance of object from the lens = u = ∞

distance of image from the lens, v = - 80 cm

Use lens formula

`(1)/(f)=(1)/(v)-(1)/(u)`

`(1)/(f)=(1)/(-80)-(1)/(\infty )`

So, f = - 80 cm

Power of the lens is the reciprocal of the focal length of the lens.

P = 100/f

where, f is the focal length when it is measured in the units of cm.

P = - 100 / 80 = - 1.3 Dioptre

Thus, option (a) is correct.