Question

Two thin lenses, one of focal length +60 cm and the other of focal length – 20 cm are put in contact. The combined focal length is –

• A. – 30cm
• B.
  • 30 cm
• C. – 15 cm
• D.
  • 15 cm

Answer 1

Answer: (A)

– 30cm

Answer 2

= $\frac { 1 } { + 60 }$ + $\frac { 1 } { - 20 }$ = $\frac { 1 - 3 } { 60 }$

\ F = – 30 cm