: a curved piece of glass or plastic used singly or combined in eyeglasses or an optical instrument (as a microscope) for forming an image by focusing rays of light
: a device for directing or focusing radiation other than light (as sound waves, radio microwaves, or electrons)
: a highly transparent biconvex lens-shaped or nearly spherical body in the eye that focuses light rays entering the eye typically onto the retina, lies immediately behind the pupil, is made up of slender curved rod-shaped ectodermal cells in concentric lamellae surrounded by a tenuous mesoblastic capsule, and alters its focal length by becoming more or less spherical in response to the action of the ciliary muscle on a peripheral suspensory ligament
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