a. Principal axis:
The line joining both the centers of curvature of a lens is called the principal axis.
b. Optical center:
The geometrical center of a lens is known as its optical center.
c. Center of curvature:
The center of the sphere from which the lens has been cut is called the center of curvature.
d. Focal length:
The distance between principle focus and optical center is called the focal length.
e. Far point:
The distance point from the eye seen clearly is called the far point.
f. Near point:
The nearest point up to which an object clearly is called near point.
g. Accommodation of an eye:
The ability of an eye to focus the image of objects at various distances on the retina by changing the focal length of the eye lens is called accommodation of an eye.
h. Range of vision:
The distance between the far point and the near point is called the range of vision.