Ultra-Violet Glasses

Abstract

Window glass is an important building material which has been manufactured from ancient times to keep out the weather and to let in the light. There are three necessary properties of any window glass: it must be:— 1. Transparent to visible light; 2. Of good quality—free from defects; 3. Permanently resistant to the weather. Glass is remarkably transparent to visible light: sheet glass and polished plate transmit 90 per cent of visible radiation. The question then arises, is this the only kind of radiation that we want glass to transmit? What other kinds are there which reach us from the sky, and which of them do we want to be transmitted? Visible light is energy propagated by electromagnetic wave-motion, and its properties are determined by its wave-length, as is true of all such radiation. The range of the electromagnetic wave-length is enormous. We have the wireless waves, thousands of metres in wave length; the beam wireless waves of about 30 metres; the infra-red radiation of the order of 1/1000th of a millimetre; then visible light, whose wave-length is measured in Angstrom units, from 7,600Å to 3,900Å; below these the ultra-violet; then X radiation; radium gamma rays—a small fraction of an Angstrom unit; and cosmic rays, which are far shorter still. Some of these forms of radiation reach the earth's surface through space, and some can only be produced artificially. The ultraviolet, we know, can be produced by the arc lamp, and its existence shown by fluorescent materials.