A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination works as convex lens of focal length 50 cm. The power of concave lens is

Question

A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination works as convex lens of focal length 50 cm. The power of concave lens is (A) P=-3.0 D (B) P=+3.0 D (C) P=-0.3 D (D) P=+0.3 D

Tardigrade Admin · Accepted Answer

Power of the combination P=P1+P2 =P text concave +P text convex ⇒ (100/50)=P text concave +(100/20) or 2=P text concave +5 or P text concave =2-5=-3.0 D

Q. A concave lens is kept in contact with a convex lens of focal length $20 c m$ . The combination works as convex lens of focal length $50 c m$ . The power of concave lens is

$P = - 3.0 D$

$P = + 3.0 D$

$P = - 0.3 D$

$P = + 0.3 D$

Power of the combination
$P = P_{1} + P_{2}$
$= P_{concave} + P_{convex}$
$\Rightarrow \frac{100}{50} = P_{concave} + \frac{100}{20}$
or $2 = P_{concave} + 5$
or $P_{concave} = 2 - 5 = - 3.0 D$

Q. A concave lens is kept in contact with a convex lens of focal length 20cm. The combination works as convex lens of focal length 50cm. The power of concave lens is

P=−3.0D

P=+3.0D

P=−0.3D

P=+0.3D