Q. If two +5 D, lenses are mounted at some distance apart, the equivalent power will always be negative, if the distance is
A) Greater than 40 cm
B) Equal to 10 cm
C) Equal to 10 cm
D) Less than 10 cm
Explanation
(a) When two lenses are separated by some distance $x$, then equivalent power
$P = P₁ + P₂ - xP₁P₂$
$\therefore P = 5 + 5 - x \times 5 \times 5$
$or\ P = 10 - 25x$
Power $P$ will be negative, if 10-25x will be negative
$ie,\ \ \ \ 25x > 10$
$or\ \ x > \frac{10}{25}$
$or\ \ x > \frac{10}{25} \times 100\ cm$
$or\ \ x > 40\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.