Question

Why the focal length of convex lens is positive and the focal length of concave lens is negative?

Answer 1

Dear student

In a convex lens , since the parallel beam of light travelling parallel to the principal axis actually meets at a point(second focus) , the focus is called real focus. The focal length is counted towards the direction of movement of light and according to the sign convention, distance towards the movement of light is counted in positive hence its focal length is taken as positive.

In a concave lens , since the parallel beam of light travelling parallel to the principal axis appear to diverge from this point(second focus) , the focus is called virtual focus. The focal length is counted opposite to the direction of movement of light and according to the sign convention, distance opposite towards the movement of light is counted as negative and its focal length is taken as negative.

Regards