Q. Two lenses of power – 15D and +5 D are in contact with each other. The focal length of the combination is
A) -20 cm
B) -10 cm
C) +20 cm
D) +10 cm
Explanation
(b) Power of lens is reciprocal of its focal length. Power of combined lens is
$P = P₁ + P₂$
$= - 15 + 5 = \ - 10\ D$
$\therefore f = \frac{1}{P} = \frac{100}{- 10}cm$
$f = \ - 10\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.