1. Main types of lenses

Lenses are solids made of some transparent material, with curved surfaces. The two main types of lense are the CONVEX and the CONCAVE lens. Convex lenses are familiar objects, since they constitute the optical element of "magnifying glasses".

A CONVEX LENS can be considered as being made up of an infinite number of prisms (see the diagram above, left) through which the rays of light are deviated (diagram above, on the right, ). The rays which pass through the center of the lens are not deviated, while the ones passing through near the edge of the lecs will be deviated the most. The result is that all parallel light rays passing through a convex lens will be deviated through one single point F, called the FOCAL POINT or PRINCIPAL FOCUS of the lens. For this reason, a convex lens is said to be a CONVERGENT LENS. Convergent lenses are thicker in the center than at the edges. (Note for educators)

Similarly, we can also imagine a CONCAVE LENS to be made up of an infinite number of prisms. Parallel rays falling onto the lens as shown in the above diagram will be refracted and will diverge, as if the rays came from the point F, the focal point of the lens. A concave lens is therefore said to be a DIVERGENT LENS. Divergent lenses are thinner in the center than at the edges.

2. Some terms used in connection with lenses

Principal axis: A line which passes through the center of the lens, perpendicular to the lens surface. (Lines X-Y in the diagrams on the left illustrate the principal axes of the lenses.)

Optical centre: This is a point on the principal axis of a lens through which light passes without undergoing any deviation. In other words, a ray of light passing through the optical center will not change its direction. For thin lenses whose faces have the same curvature, this point, marked O in the diagram, is in the center of the lens.

Principal focus or focal point: This is a point, marked F in the diagram, to which all rays parallel to the principal axis converge (in the case of a convex lens), or (in the case of a concave lens) from which the rays appear to diverge.

Focal length: This is the distance between the optical centre and the principal focus. In the diagram, it is the distance OF.

Principal focal plane: An imaginary plane located at the principal focus, perpendicular to the principal axis.

What about gravitational lenses?

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3. Additional questions

Note for educators:

The discussion assumes that be are dealing with thin lenses, whose thickness are small in relation to other distances, and monochromatic light, thus eliminating chromatic aberration.

Gravitational lenses

Einstein's theory of relativity predicts that a massive object, such as the sun, can "bend" the path of light. This was observed to be the case, and we find that very distant light sources, such as quasars () can form images if the light passes close to galaxies or clusters of galaxies in distant space.

See http://imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html.