Question

If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power?

• A. #ERROR!
• B. -0.75 D
• C. #ERROR!
• D. -6 .5 D.

Answer 1

Answer: (B)

-0.75 D

Answer 2

Focal length $(f_1)$ = 80 cm and $(f_2)$ = - 50 cm
(Minus sign due to concave lens)
Power of the combination (P)
$P_1 + P_2 = \frac{100}{f_1} +\frac{100}{f_2} = \frac{100}{80} - \frac{100}{50} = -0.75 D$