Define the term principal foci of a concave lens and show them with the help of proper diagrams.

What happens to the size and intensity of the image if a part of a lens is covered ?

Draw a diagram to represent the second focus of a convex lens.

Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass slab and two triangular glass prisms.