Two plano-convex lenses of focal lengths 20 cm and 30 cm are placed together to form a double convex lens. The final focal length will be

Question

Two plano-convex lenses of focal lengths 20 cm and 30 cm are placed together to form a double convex lens. The final focal length will be (A) 12cm (B) 60cm (C) 20cm (D) 30cm

Tardigrade Admin · Accepted Answer

Equivalent focal length (1/F)=(1/f1)+(1/f2) =(1/20)+(1/30) ∴ F=(20 × 30/20 + 30) =(600/50)=12 text cm

Q. Two plano-convex lenses of focal lengths $20 c m$ and $30 c m$ are placed together to form a double convex lens. The final focal length will be

$12 c m$

$60 c m$

$20 c m$

$30 c m$

Equivalent focal length
$\frac{1}{F} = \frac{1}{f _{1}} + \frac{1}{f _{2}}$
$= \frac{1}{20} + \frac{1}{30}$
$∴ F = \frac{20 \times 30}{20 + 30}$
$= \frac{600}{50} = 12 cm$

Q. Two plano-convex lenses of focal lengths 20cm and 30cm are placed together to form a double convex lens. The final focal length will be

12cm

60cm

20cm

30cm