1. A ground or molded piece of glass, plastic, or other transparent material with opposite surfaces either or both of which are curved, by means of which light rays are refracted so that they converge or diverge to form an image.
2. A combination of two or more such pieces, sometimes with other optical devices such as prisms, used to form an image for viewing or photographing. Also called "compound lens".
3. A device that causes radiation other than light to converge or diverge by an action analogous to that of an optical lens.
4. A transparent, biconvex body of the eye between the iris and the vitreous humor that focuses light rays entering through the pupil of the eye to form an image on the retina.
5. The transparent part of the eye, behind the pupil.

The lens was named after the lentil bean because it resembled it in shape and size

A lens is the transparent structure inside the eye that focuses light rays onto the retina which is the nerve layer that lines the back of the eye, senses light and creates impulses that go through the optic nerve to the brain.

In addition to the lens, the eye has a number of other components including the cornea, iris, pupil, retina, macula, optic nerve, and vitreous.

• The cornea is the clear front window of the eye that transmits and focuses light into the eye.
• The iris is the colored part of the eye that helps regulate the amount of light that enters the eye.
• The pupil is the dark aperture in the iris that determines how much light is let into the eye.
• The retina is the nerve layer that lines the back of the eye, senses light and creates impulses that travel through the optic nerve to the brain.
• The macula is a small area in the retina that contains special light-sensitive cells and allows us to see fine details clearly.
• The optic nerve is the nerve that connects the eye to the brain and carries the impulses formed by the retina to the visual cortex of the brain.
• The vitreous humor is a clear, jelly-like substance that fills the middle of the eye.

In ultrasonography, a lens used to focus or to diverge a sound beam which may be simulated by the electronic manipulations of signals: Acoustic lenses are made of a material by which the speed of sound differs from that in the surrounding medium and the acoustic lenses may be solid, liquid, or gas.

A lens used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general: The reading addition of a bifocal lens may be cemented to the lens, fused to the front surface, or ground into a one-piece form.

Other bifocal lenses are the flat-top Franklin type, or blended invisibly.

A system composed of several waveguides that are symmetrically arranged with a related excitation medium in order to become excited with equal amplitudes and phases to in order to provide a net gain in energy.

Elements in a single viewing aperture in which there are curvature centers on both of its surfaces: The meniscus lenses may lie on the same side of the lens so that both arched areas are concave or both are convex or the meniscus lens have a spherical concave curve on one side and a spherical convex curve on the other side.

1. A lens for near and distant vision in which the reading portion is a continuously variable curve.
2. A lens whose power increases continuously and regularly in a downward direction, avoiding the discontinuity of bifocal and trifocal lenses.

A lens with three parts of different refracting powers: the upper for distant sights, the middle for intermediate views, and the lower for near visions.