A lens is defined as a piece
of transparent, optical material having one or two spherical
surfaces (whose central axes coincide).
A lens can have one spherical surface
and one plane surface or it can have two spherical surfaces.
When the surface is spherical, then it has some finite radius
of curvature which can be measured. However if one surface
is plane, its radius is considered to be infinite.
Lenses are divided in two categories:
Converging lens or convex lens
Diverging lens or concave lens.
Each of the above category can have multiple types associated
with it. Convex lenses can be of subtype as plano convex
or concavo convex, or double convex lens similarly concave
lenses can be of plano concave or convexo concave.
A convex lens: a piece of transparent, optical material,
having one or two spherical surfaces such that it is thicker
in the middle and tapering at the edges .
All of the above are thicker in the middle and tapering at
their edges.
The analytical relation between the object distance, the
image distance and the focal length is given by
1/f=1/do + 1/di ; where do is the object distance and
di is the image distance.
Also explained in the animation are common terms like
 Principal axis,
 radius of curvature,
 optical center,
 focus ,
 focal length.
A detailed description of how to construct a ray diagram
for both types of lenses is shown, another area that is definitely
confusing to beginners and is clearly explained through animation
is why refraction is only taken to occur once for the ray
traveling through the lens while in reality the ray undergoes
medium change twice in the lens ?
Explained in more detail in the section of very popular "explanatory
notes" are the key concepts about
 Power of a lens
 Formation of image with respect to lens type
 Magnification formula
 Sign convention
