Question

A man who cannot see clearly beyond 5 m wants to see stars clearly. He should use a lens of focal length

• A. – 100 m
• B.
  • 5 m
• C. – 5 m
• D. Very large

Answer 1

Answer: (C)

– 5 m

Answer 2

For lens u = want’s to see = $\infty$

v = can see = – 5 m

∴ From $\frac { 1 } { f } = \frac { 1 } { v } - \frac { 1 } { u }$ $\Rightarrow \frac { 1 } { f } = \frac { 1 } { - 5 } - \frac { 1 } { \infty }$ $\Rightarrow f = - 5 m$