Define principal focus of Convex lens and that of a concave lens.

Hint: A convex lens also known as a converging lens is thick in the middle and thin at the edges. It converges the light rays coming from a source on the principle focus. A principle focus of a convex lens is defined as the point on the principal axis on which the light rays coming parallel to the principal axis converge after getting refracted from the convex lens.

Complete answer:

 
A concave lens also known as the diverging lens is a type of lens that is thin in the middle and thicker at the edges. It diverges the rays of light coming parallel to the principal axis. The principal focus of a concave lens is that point on the principal axis on which the rays of light that get diverged after refraction through the lens, when produced backwards, tend to meet. The refracted rays appear to diverge from the Principal Focus of a concave lens.

Additional Information: The distance between the Lens and the principle focus is called the focal length. By convention the measurements on the right side of the lens/mirror are taken positive and the ones on the left side are taken negative. Similarly, the distances above the principle axis are taken as positive and below it are negative.
Note: For a converging lens the rays would converge on the focus after refraction and for a diverging lens the rays would appear to diverge from the principal focus after refraction.