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Large speculum metal mirrors are hard to manufacture, and the alloy is prone to tarnish, requiring frequent re-polishing. However, it was the only practical choice for large mirrors in high-precision optical equipment between the mid-17th and mid-19th centuries, before the invention of glass silvering .
In the early 10th century, the Persian scientist al-Razi described ways of silvering and gilding in a book on alchemy, [citation needed] but this was not done for the purpose of making mirrors. Tin-coated mirrors were first made in Europe in the 15th century. The thin tinfoil used to silver mirrors was known as "tain". [5]
This roof edge diffraction effect may also be seen as a diffraction spike perpendicular to the roof edge generated by bright points in the image. In technical optics, such a phase is also known as the Pancharatnam phase , [ 4 ] and in quantum physics an equivalent phenomenon is known as the Berry phase .
The removal of oxidization (tarnish) from metal objects is accomplished using a metal polish or tarnish remover; this is also called polishing. To prevent further unwanted oxidization, polished metal surfaces may be coated with wax, oil, or lacquer. This is of particular concern for copper alloy products such as brass and bronze. [2]
A mirror reflecting the image of a vase A first-surface mirror coated with aluminium and enhanced with dielectric coatings. The angle of the incident light (represented by both the light in the mirror and the shadow behind it) exactly matches the angle of reflection (the reflected light shining on the table). 4.5-metre (15 ft)-tall acoustic mirror near Kilnsea Grange, East Yorkshire, UK, from ...
It is much faster to set up than the standard Foucault knife-edge test. The Ronchi test differs from the knife-edge test, requiring a specialized target (the Ronchi grating, which amounts to a periodic series of knife edges) and being more difficult to interpret. This procedure offers a quick evaluation of the mirror's shape and condition.
The optical glass available at the time tended to develop a tarnish on its surface with age, due to chemical reactions with the environment. Rayleigh tested some old, slightly tarnished pieces of glass, and found to his surprise that they transmitted more light than new, clean pieces. The tarnish replaces the air-glass interface with two ...
Gilding gives an object a gold appearance at a fraction of the cost of creating a solid gold object. In addition, a solid gold piece would often be too soft or too heavy for practical use. A gilt surface also does not tarnish as silver does. Modern gilding is applied to numerous and diverse surfaces and by various processes.