Lenses

A lens is a piece of transparent material that can focus a transmitted beam of light. This is usually bounded by two spherical surfaces, or a spherical and a plane surface.  

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      A piece of transparent  substance (commonly glass, plastic, quartz etc.) bounded by two surfaces of regular curvature is known as "Lens". A lens is used to change the curvature of wavefronts so that light may be focused to a desired position.

Basically thin lenses fall into two categories:

Converging or Convex

Diverging or Concave

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       A convex lens is thicker in the middle and thinner at the edges, converges light rays towards its optical axis, so that a beam of parallel rays converges at a point F (shown in figure given below). For example in bright sun light, a convex lens may produce a spot of light intense enough to ignite paper.

  Fig   Convex lens and its Principal Focus or Focal Point

    A concave lens which is thinner in the middle and thicker at the edges bends rays outward from its optical axis (see figure below)

  Fig   Concave lens and its Principal Focus or Focal Point

       The point F to which the rays are brought to focus is called the principal focus or the focal point, and the distance between the optical center of the lens and its principal focus is called the focal length f. Conventionally f is taken to be positive for convex lenses and negative for concave lenses.