| Nicol prism | <physics> A prism, used for polarizing or analysing light, made by cementing together, with Canada balsam, two pieces of calcite in such a way that the extraordinary ray from the first piece passes through the second piece while the ordinary ray is reflected to the side into an absorbing layer of black paint. When two Nicol prisms are crossed, therefore, no light passes through. A polarizing prism made of two pieces of calcite specially cut, ground, polished and cemented. A transmitted beam splits into two polarized components, one of which is refracted into and absorbed by the asphalt mount. The remaining polarized beam is transmitted. Modifications of this prism are common. The Nicol prism (nicol) has generally been superseded by a polarizing film such as Polaroid. See: polars, polarizer, analyser. (05 Aug 1998) |
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| tidal prism | The change in the volume of water covering an area, such as a wetland, between a low tide and the subsequent high tide. (09 Oct 1997) |
| Fresnel prism | A prism composed of concentric annular rings. (05 Mar 2000) |