Question

If a combination of 3 convex lens is arranged in the shown manner the focal length of lens I is + 10 cm, lens II is 20 cm and lens III is 40 cm. Calculate the total power of the combination of lens?

• A. – 0.175 D
• B. + 1.75 D
• C. +17.5 D
• D. + 0.75 D

Answer 1

The correct option is C

+17.5 D

Let $P$ be the power of lens and $f$ be the focal length of the lens.
Power of a lens = $\frac{1}{f\left(inm\right)}$
focal length of lens I $=+10cm=0.1m$ (as per sign conventions)
focal length of lens II $=+20cm=0.2m$ (as per sign conventions)
focal length of lens III $=+40cm=0.4m$ (as per sign conventions)
Total power is the combination of these three lenses.

Total Power = $P_I$ + $P_{II}$ + $P_{III}$

$P_I$ = $\frac{1}{0.1}=10D$ ; $P_{II}$ = $\frac{1}{0.2}=5D$ ; $P_{III}$ = $\frac{1}{0.4}=2.5D$

Total (P) = $10+5+2.5=17.5D$