Question

define principal focus of a Convex lens and Concave lens

Answer 1

principal focus of convex lens is light traveling parallel to principal axis of a convex Lenz after refraction meet at a point on principal axis called principal focus.

Answer 2

Answer:

Principal focus of Convex lens:

• “Principal focus of convex lens” is the point on its ‘principal axis’ to that the light rays becomes parallel to the ‘principal axis converge’ after the lens refraction.

Principal focus of Concave lens:

• ‘The concave lens’ is the diverging lens.
• When the “parallel rays of light” passes through the concave lens where the ‘refracted rays diverge’ so that they can appear to come from a single point called “the principal focus”.  
• There is some distance between “the principal focus” and “the centre of the lens”, which is called ‘the focal length’.