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The study of physical characteristics of earth and the distribution and effects of life on earth, especially that of humans.

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Astigmatic power lens

Eyewear; Optometry

Lens bringing a paraxial pencil of parallel light to two separate line foci mutually at right angles and hence having vertex power in only the two principal meridians.

Astronomical telescope

Eyewear; Optometry

Compound optical system, afocal in normal adjustment, consisting of a positive objective element or group and a positive ocular element or group forming a magnified, inverted ...

Condensing lens

Eyewear; Optometry

A convex lens used to condense the illuminating beam into the patient's eye and to form a real inverted image of the retina thus illuminated.

Concave lens

Eyewear; Optometry

Lens which causes parallel incident light to diverge from a virtual focus. Also known as minus-power lens.

Abbe number

Eyewear; Optometry

Mathematical expression for determining the correction for chromatic aberration of an optical material of component.

Astigmatic difference

Eyewear; Optometry

Vertex power in the second principal meridian minus that in the first principal meridian.

Optical drive


An optical drive is an electronic device used to seek, read, and write data on an optical disc. An optical drive may reside in an optical library or as a stand-alone unit.