A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. Lenses are made from materials such as glass, plastic, and quartz. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis. Lenses are used in optical instruments such as cameras, microscopes, telescopes, and binoculars.
The word lens comes from the Latin word for lentil, because a double-convex lens is shaped like a lentil seed. Lenses are classified by their shape and the curvature of their surfaces. A Lens can be either convex (bulging outward) or concave (hollowed out). Most lenses used today are curved in order to focus light onto or into an image plane; however, some special lenses may have planar surfaces.
The two main types of curving are spherical and aspheric. Spherical curves have at least one radius of curvature that is constant throughout the surface; aspheric curves have varying radii of curvature across the surface. The degree of curvature is measured by the radius of curvature (R), which is the distance from the center point of the curve to its edge. For example, an R_1/2 lens has twice the curvature as an R_1 lens at any given point on its surface. Curvatures can also be negative; this occurs when part or all of the surface curves inward like a bowl (a concave mirror). The term “power” is sometimes used to describe the amount of bending that takes place with a particular curve; thus, an R_2 curve has twice the powerof an R_1 curve at any given point.
Lenses can be categorized based on how they modify Incident light:
Refractive indexes: n > 1
Convex lenses: converge incident parallel rays
biconvex: both sides convex
plano-convex: one flat side
meniscus:one convex side ,one concave side with steeper curve than other convex-sided lenses
Concave lenses: diverge incident parallel rays
biconcave :both sides hollowed out(curved inward)
plano-concave :one flat side
aberration : deviation from perfect image formation due to imperfections in eitherthe optics or object itself .There are 3 main types..chromatic(color related ),spherical(due to unequal bending ),and comatic(offsets occur along lines perpendicular to chief ray )