Define the principal focus of a lens.

Draw ray diagram to shown the principal focus of a (i) concave mirror, (ii) convex mirror.

Define the principal focus of a spherical mirror.

Define principal focus of a lens.

Draw the course of a rays for tracing image of an object held between pole and principal focus of a concave mirror. State the position and nature of image formed. or Draw the course of rays for tracing image of an object held between optical centre and principal focus of a convex lens. State the nature of image formed.

What do you mean by principal focus of a concave mirror ?

Define principal focus.

Principal focus of concave lens:

A spherical mirror in which reflecting surface is towards the centre of the sphere of which the mirror is a part, is called concave mirror, i.e., reflection of light for this mirror occurs at concave surface or bent-in surface. The rays of light incident on concave mirror in a direction parallel to the principal axis, actually meet at a single point F on the principal axis of the mirror. This point is called principal focus of the mirror. Read the above passage and answer the following questions : (i) Is principal focus of a concave mirror, a real point or a virtual point? (ii) What happens in case of a convex mirror ? (iii) Our teachers and parents advise us to stay focussed. What does it imply?