Definition: Material that appears of different colour when seen with transmitted light and when seen by reflected light. * Literally: two-colour. Adjective: (2) ~ mirror: one used to reflect light selectively, according to its wavelength. * Also known as diathermic mirror. (3) ~ polarizer: one that transmits light that is almost completely plane polarized. * The transmitted intensity is half that of the incident intensity. (4) ~ bulb: one that uses a dichroic mirror to reflect visible light but but allow passage of infra-red light. * Compare with half-mirror. (5) ~ fog: thin, fine deposit of molecular silver on film or surface e.g. from exhausted fix. * The colour of the deposit appears to change with viewing angle due to diffraction.

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