iBT TOEFL Topic Vocabulary Breakthrough Unit 043

iBT TOEFL Topic Vocabulary Breakthrough Unit 043
Author: Li Xiaolai

43. Photography is based on principles of light, optics, and chemistry. For the purpose of producing a photograph, these silver salts are distributed in gelatin to make a mixture called an emulsion, which is applied to film or another supporting material in a thin layer. When the emulsion is exposed to light, the silver halide crystals undergo chemical changes and, after further processing, an image becomes visible. The stronger the light that strikes the crystals, the denser or more opaque that part of the film becomes. Most types of film produce a negative image, from which a positive final copy can be printed on sensitized paper. The dense (or dark) areas of the negative translate into light areas on the final photograph. Almost all modern photography relies on this negative-to-positive process.

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