Luminous and Illuminated Bodies
In general, vision is due to the stimulation of the optic nerves in the eye by light either directly from its source or indirectly after reflection from other objects. A luminous body, such as the sun, another star, or a light bulb, is thus distinguished from an illuminated body, such as the moon and most of the other objects one sees. The amount and type of light given off by a luminous body or reflected by an illuminated body is of concern to the branch of physics known as photometry (see also lighting). Illuminated bodies not only reflect light but sometimes also transmit it. Transparent objects, such as glass, air, and some liquids, allow light to pass through them. Translucent objects, such as tissue paper and certain types of glass, also allow light to pass through them but diffuse (scatter) it in the process, so that an observer cannot see a clear image of whatever lies on the other side of the object. Opaque objects do not allow light to pass through them at all. Some transparent and translucent objects allow only light of certain wavelengths to pass through them and thus appear colored. The colors of opaque objects are caused by selective reflection of certain wavelengths and absorption of others.
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