Q. An eye specialist prescribes spectacles having a combination of convex lens of focal length $40\ cm$ in contact with a concave lens of focal length $25\ cm$. The power of this lens combination in dioptres is
A) $+ 1.5$
B) $- 1.5$
C) $+ 6.67$
D) $- 6.67$
Explanation
(b) Power of convex lens $P_{1} = \frac{100}{40} = 2.5\ D$
Power of concave lens $P_{2} = - \frac{100}{25} = - 4\ D$
Now $P = P_{1} + P_{2} = 2.5\ D - 4\ D = - 1.5\ D$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.