Question

What is the difference between a double convex and a bi-convex lens?

Answer 1

In both cases, the two surfaces are convex towards the air. In the case of a double convex lens, the two surfaces may have any values for the two radii of curvature of the two surfaces while in the bi-convex lens the radii of curvature of the two surfaces, has the same value. Thus, every bi-convex lens is a double convex lens about every double convex may not be bi-convex.