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

Question

If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm combined together, what will be their result power ? (A) + 0.65 D (B) -0.65 D (C) + 0.75 D (D) -0.75 D

Tardigrade Admin · Accepted Answer

f1=+80 cm, f2=-50 cm Resultant focal length F =(f1 f2/f1+f2) =(80 ×-50/80-50)=(-4000/30) =-(4000/3) cm Resultant power P=(100/F)=(100/-400 / 3) =-(3/4) =-0.75 D

Q. If a convex lens of focal length $80 c m$ and a concave lens of focal length $50 c m$ combined together, what will be their result power ?

+ 0.65 D

-0.65 D

+ 0.75 D

-0.75 D

$f_{1} = + 80 c m, f_{2} = - 50 c m$
Resultant focal length
$F = \frac{f _{1} f _{2}}{f _{1} + f _{2}}$
$= \frac{80 \times- 50}{80 - 50} = \frac{- 4000}{30}$
$= - \frac{4000}{3} c m$
Resultant power $P = \frac{100}{F} = \frac{100}{- 400/3}$
$= - \frac{3}{4}$
$= - 0.75 D$

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