Question

A convex lens of focal length 25cm and a concave lens of focal length 10cm are placed in close contact with each other. Calculate the lens power of this combination

Answer 1

Given,

The focal length of the convex lens = 25 cm = 0.25 m

We know,

$Power=\frac{1}{f}$

Or, $Power=\frac{1}{0.25}=4D$

Power of the convex lens $P_1=4D$

The focal length of the concave lens = -10 cm = -0.1 m

We know,

$Power=\frac{1}{f}$

Or, $Power=\frac{1}{-0.1}=-10D$

Power of the concave lens $P_2=-10D$

Now,

Power of the combination = $P_1+P_2=(1-10)=-6D$

Hence , the power of the combination is -6 D.