The distance between principal focus and optical center of the lens is ________.

The focal length of a concave mirror is $50\ cm.$ Where an object be placed so that its image is two times and inverted

Fill in the following blanks with suitable words :

(a) Parallel rays of light are refracted by a convex lens to a point called the ______

(b) The image in a convex lens depends upon the distance of the ____ from the lens.

The geometrical centre of a spherical mirror is called its pole.

The distance between the extreme points on the periphery of the mirror is called: