The two surfaces of a biconvex lens have the same radii of curvatures. This lens is made of glass of refractive index $1.5$ and has a focal length of $10$ cm in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano-convex lenses. Then they are put in contact with their convex surfaces touching each other. If this combination lens is immersed in water (refractive index$=4/3$), its focal length (in cm) is :

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

The maximum magnification that can be obtained with a convex lens of focal length $2.5\ cm$ is ( the least distance of distinct vision is $25\ cm$)

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