Question

Two lens of +3D and -2D power are combined to form an equivalent lens. What is the focal length of equivalent lens ?

(A) 3 metre

(B) 5 metre

(C) 1 metre

(D) 2 metre

Answer 1

Correct option is (C) 1 metre

\(P = \frac 1f\) when f is in m

\(f = \frac 1P = \frac 1{P_1 + P_2}\)

\(f = \frac 1 {3 + (-2)} = \frac 11 = 1 m\)