Question

How to calculate focal length of a convex lens without actual calculation?

Answer 1

Answer:

The distance of  the image from the lens is the focal length of the convex lens.

Explanation:

Place the convex lens in such a position that sunlight is perpendicular to the lens. In such a position sunlight would be parallel to the optical axis of the lens.

Place a piece of paper behind and parallel to the lens and adjust its distance from the lens so as to obtain a point image of sunlight.

Because  the sunlight converge at the focal point after passing through a convex lens.