Question

A convex lens of focal length 40 cm is an contact with a concave lens of focal length 25 cm. The power of combination is

• A. – 1.5 D
• B. – 6.5 D
• C.
  • 6.5 D
• D.
  • 6.67 D

Answer 1

Answer: (A)

– 1.5 D

Answer 2

By using $\frac { 1 } { F } = \frac { 1 } { f _ { 1 } } + \frac { 1 } { f _ { 2 } }$ ⇒ $\frac { 1 } { F } = \frac { 1 } { + 40 } + \frac { 1 } { - 25 }$

⇒ , hence $P = \frac { 100 } { f ( c m ) } = \frac { 100 } { - 200 / 3 } = - 1.5 \mathrm { D }$