A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

Explanation

Here, `f_1=40cm , f_2= -25cm,P=?`
Clearly, `P_1=(100)/(f_(1))=(100)/(40)=2.5D`
and `P_(2)=(100)/(f_(2)) = (100)/(-25)= -4.0D`
As `P=P_1+P_2, P=2*5 - 4*0= -1.5D`