Special kinds of glass can be made if other chemicals, such as metal oxides are added. Borosilicate glass is made from the chemicals silica and boric oxide and is usually known by the trade name Pyrex. Pyrex is used to make casserole dishes, chemical glassware and industrial flasks and pipes because it does not crack when heated.
Metal oxides, such as zinc, lead and magnesium oxide, are added to make high quality optical glass for the lenses in cameras, microscopes, telescopes and some spectacles. Optical glass is hard to form and expensive to produce. It must be completely transparent so that light passes directly through it, without distortion.
Binoculars and microscopes use optical glass to refract light and make objects look bigger.
The Hubble space telescope mirrors are extremely smooth, with precisely shaped reflecting surfaces.
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