Question

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

(a) + 25 cm

(b) – 25 cm

(c) + 50 cm

(d) – 50 cm

I had been asked this question in final exam.

I would like to ask this question from Optical Instruments topic in division Ray Optics and Optical Instruments of Physics – Class 12

Answer 1

Right choice is (c) + 50 cm

Best explanation: By thin lens formula,

\(\frac {1}{f} = \frac {1}{v} – \frac {1}{u}\)

\(\frac {1}{f} = \frac {1}{(-50)} – \frac {1}{(-25)}\)

f = +50 cm.