1. Review of geometrical optics
Light travels through space in straight lines. If a ray of light meets a body in its passage through space one of three things may happen:
It may be absorbed.
It may be reflected.
It may be transmitted.
When the light transmits from one medium into another medium of different density, it is called to be refracted.

2. Principles of vergence
As applied to light rays, the term vergence describes the direction of a ray as it passes between some luminous point to a lens.
Vergence power is the reciprocal of the distance from the lens to the point of convergence of the light.
Light rays that moving away from each other are termed divergent.
Light rays that are moving toward each other are termed convergent.
Parallel light rays have zero vergence.

3. Divergent rays convergent rays parallel rays

4. Vergence
1. Light rays emanating from a point source of light are divergent. 2. Convergent light rays does not usually occur in nature but are the result of the action of an optical system (e.g., a lens). 3. Light rays emanating from the sun are essentially parallel and have zero vergence. -Power (or vergence power) describes the ability of a curved lens to converge or diverge light rays. By convention, divergence is expressed in minus power and convergence is expressed in plus power. - Diopter is the unit of measurement of the refractive power of a lens and is abbreviated (D).

5. Type of lenses Lenses may be sphere, cylinders and spherocylinders.
Spherical lens has the same curvature over its entire surface, and thus the same refractive power in all meridians.
Convex spherical lenses converge light rays and are called plus lenses, while concave spherical lenses diverge light rays and called minus lenses.
The focal point of a plus lens is that position where parallel light rays that have passed through the lens and converge to form an image.
The focal point of a minus lens is the point which parallel light rays entering the lens appear to diverge.
In the case of convex, or plus lenses using the mathematical formula:
D=1/f

6. Type of lenses
Cylindrical lenses have vergence power in only one meridian, that perpendicular to the axis of the cylinder. They have no power in the meridian parallel to the axis. These lenses focus light rays to a line.
Spherical and cylindrical lenses can be combined in one a lens to form a spherocylindrical lens, also known as compound lens or toric lens. These lenses focus light rays in two line foci.
The shape of the light rays as they are focused by the spherocylinderical lens is called the conoid of strum.
Between the two line foci produced by the conoid of strum is a point called the circle of least confusion, which represents the point of the best overall focus for the lens.