Question

A convex lens focal 25 m and a concave lens of focal length 10 cm are placed in calculate the lens power of combination

Answer 1

A convex lens focal 25 m and a concave lens of focal length 10 cm.

We have to find the power of lens of the combination.

Power of lens is defined as the reciprocal of focal length. And we know that focal length of concave lens is negative and focal length of convex lens is positive.

⇒ f (convex lens) = 25 cm f (concave lens) = -10 cm

Now,

P (convex lens) = 1/f = 100/25 m = 4 D

P (concave lens) = 1/f = 100/(-10) m = -10 D

As per given condition, we have to find the power of lens of combination (concave and convex lens)

Let's denote the power of lens by P and power of lens for convex lens by P' & power of lens of concave lens by P".

From above data we can say that -

• P' = 4D and P" = -10D

Power of lens (combination) = Power of lens of convex lens + Power of lens of concave lens

P = P' + P"

P = 4D + (-10D)

P = 4D - 10D

P = -6D