Question

odd one out and give reason-pole,principal focus,centre of curvature and principal axis

Answer 1

Dear student,
Odd one is principal axis because pole, principal focus, centre of curvature all are the points lying on the principal axis. Principal axis is not a point but it is a line therefore principal axis is odd among others.
Pole - It is defined as a point where principal axis meets the mirror.
Principle focus - It is defined as a point on the principal axis where rays passing through the lens or mirror meets.
Centre of curvature - It is defined as the centre of the lens or mirror. Lens or mirror is curved therefore it may form an imaginary sphere whose centre is the centre of curvature which is also imaginary.
Principle axis - It is defined as the line passing through the pole, centre of curvature of mirror or lens.

Regards