Question

How do you find the rough focal length of a convex lens? Is the same method applicable to a concave lens? Which factors determine the focal length of a lens? A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with each other. What is the power of this combination? Also, calculate focal length of this combination?

Answer 1

power of this combination is

1/25+1/20 =4/100+ 5/100 =9/100