Question

a convex lens of focal length 40cm and a concave lens of focal length 200cm are placed in contact with each other . Find the power of the combined lens

Answer 1

focal length of Convex lens= 40cm
and the focal length= - 200cm(by sign convention)
Now we know that
Power of the combined lens= Power of Convex lens+ Power of concave lens
1/p=1/40 + 1/ - 200
=-5+1/-200
=-4/-200cm
=1/50cm
Power of lens=50cm
=0.5m