Question

How do you find the focal length of a concave lens using a convex lens?

Answer 1

Use the convex lens to form a real image. Then place the concave lens in front of that image. The convex lens's image acts as an object with a negative value of do for the concave lens, which should now be able to form a real image behind it. Use the lens equation for solve for the f of the concave lense, which will be negavive