State the condition for each of the following :

(a) a lens has both its focal lengths equal.

(b) a ray passes undeviated through the lens.

Complete the following sentences —

(a) If half part of a convex lens is covered, the focal length …………… change, but the intensity of image …………….

(b) The focal length of a thin convex lens is …………… than that of a thick convex lens.

(c) A tumbler filled with water behaves as a …………… lens.

State difference between a convex and a concave lens in their —

(a) appearance, and

(b) action on the incident light.

Define the term principal axis of a lens.