Question

What focal length should the reading spectacles have
for a person for whom the least distance of distinct vision is 50 cm?

Answer 1

u = -25
v=-50
1÷f=1÷v - 1÷u
1÷-50 + 1÷25
= -1÷50
f = 50

Answer 2

Dear Student,

◆ Answer -

Convex lens of focal length 50 cm

● Explanation -

For the normal person, least distance of distinct vision is 25 cm. But given person have least distance of distinct vision as 50 cm. So u = -25 cm and v = -50 cm.

Using lens formula,

1/f = 1/v - 1/u

1/f = 1/-50 - 1/-25

1/f = -1/50 + 2/50

1/f = 1/50

f = +50 cm

Hence, focal length of reading glasses should be +50 cm. i.e. convex lens.

Thanks dear. Hope this helps you...