Focus length of a lens-
The most important characteristic of a lens is its principal focal length, or its inverse which is called the lens strength or lens "power". Optometrists usually prescribe corrective lenses in terms of the lens power in diopters. The lens power is the inverse of the focal length in meters: the physical unit for lens power is 1/meter which is called diopter
The word convex means curving out or bulging outward, as opposed to concave.
The word concave means curving in or hollowed inward, as opposed to convex.
Both of them-
Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (or double convex, or just convex) if both surfaces are convex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave.
Factors that affect focal length-
Factors that affect focal length are:
• The curvature of then lens or mirror and in case of the lens.
• The refractive index of the material the lens is made of.
The refractor is what everyone recognises as a telescope; it has a lens, called the Objective lens at the front. In some designs, particularly smaller scopes, the air spaced doublet will be replaced by a compound lens with the two elements cemented together.
As light rays pass through a lens with power, the rays are bent or refracted. In a lens with a plus power, the light rays converge or are refracted toward one another. The point at which the light rays converge is called the focal point and in a plus lens, is behind the lens surface. In a lens with a minus power, the light rays diverge or are refracted away from one another. If these rays are extrapolated or traced back toward the light source, the lines will converge and form a focal point in front of the lens