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Crown glass (optics)

From Wikipedia, the free encyclopedia

Crown glass is a type of optical glass used in lenses and other optical components. It has relatively low refractive index (≈1.52) and low dispersion (with Abbe numbers around 60). Crown glass is produced from alkali-lime silicates containing approximately 10% potassium oxide and is one of the earliest low dispersion glasses.

As well as the specific material named crown glass, there are other optical glasses with similar properties that are also called crown glasses. Generally, this is any glass with Abbe numbers in the range 50 to 85. For example, the borosilicate glass Schott BK7[1] is an extremely common crown glass, used in precision lenses. Borosilicates contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage. Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide.

BAK-4 barium crown glass has a higher index of refraction than BK7, and is used for prisms in high-end binoculars. In that application, it gives better image quality and a round exit pupil.

An achromatic doublet, which combines crown glass and flint glass.
An achromatic doublet, which combines crown glass and flint glass.

A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet. The dispersions of the glasses partially compensate for each other, producing reduced chromatic aberration compared to a singlet lens with the same focal length.

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  • Uncut Crown of Glass
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Transcription

bjbj These two objects, the uncut crown and the uncut cylinder, demonstrate the ways that window glass was made in earlier years. Here s how you make a piece of crown glass. You take a gather of glass on the end of a blowpipe, and you blow it into a bubble just as you re starting any piece of glass. You then open it up as though you were making a glass bowl, but what you do, is you reheat it, and while it s very hot, you spin it as fast as you can on the end of the blowpipe. Something called centrifugal force comes into play, and the glass simply splays outwards. And you keep twirling it on the rod until the glass is a perfectly flat disk, except for the bit at the center, because you ve got to get it off the end of the blowpipe, and in doing that, you almost always invariably leave a little scar. You can see the knob in the center, where the blowpipe was attached to the gather of glass. The problem with this method is that you re limited in the size of the panes. And so although it was popular right up through the early 19th century, because of the fact that you couldn t cut very large panes from it, the cylinder method was developed in the 17th century, and competed with the crown glass method until the 19th century when that became the standard way. And the big cylinder, which you see, was the standard for the way window glass making was done in the latter-part of the 19th century. The workman blew a very large cylinder of glass, and then it was cut and flattened out into sheets, and the sheets could be cut into large panes of glass. If you look at this model over on the right, you can see exactly how it was done. You can see the men blowing the cylinders of glass, and they have to be standing up on platforms to allow for the length of the cylinders. And then at the left-hand side of the model, you can see in the back where the back the workmen are splitting the cylinders lengthwise, and then they have to be put back into the furnace and flattened, and what you get is a large flat sheet. So you can make much larger panes by this method, but they were somewhat wavy because of the fact that they were split and flattened. If you look at Victorian windows today, you will often see that half of the window, or even the whole sash, is one piece of glass, and if you look at those, though, you can see the wavy lines. hE`T gdE`T These two objects, the uncut crown and the uncut cylinder, demonstrate the ways that window glass was made in earlier years WrightDC Normal.dot WrightDC Microsoft Word 10.0 The Corning Museum of Glass These two objects, the uncut crown and the uncut cylinder, demonstrate the ways that window glass was made in earlier years Title Microsoft Word Document MSWordDoc Word.Document.8

See also

Notes

  1. ^ The crown/flint distinction is so important to optical glass technology that many glass names, notably Schott glasses, incorporate it. A K in a Schott name indicates a crown glass (Krone in German — Schott is a German company). The B in BK7 indicates that this is a borosilicate glass composition.

External links

This page was last edited on 20 May 2021, at 14:01
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