OPTICAL PROPERTIES

The special optical characteristics of Diamond play only a small part in technology, but are, of course, a main attraction in the use of the Diamond as a Gemstone. When light enters at an angle into a solid or liquid the path changes direction, i.e. the light beam ‘bends’ at the interface, or, in a more usual terminology, it ‘refracts’. This happens because the speed of light in the solid (or liquid) is less than that in air. The ratio of the speed of light in air to that in the solid is called the refrac­tive index and this is a number to give us a measure of the extent of the refraction. In water this number is 1∙33, for glass I it is 1∙5. For Diamond it is exceptionally high, namely 2∙4. Several interesting consequences follow from this very high, refractivity of Diamond. The first concerns reflectivity. The reflecting power of a transparent surface depends on the refractive index.

Glass has a reflectivity of 4%, but Diamond is four and a half times better, with a reflectivity of 18%. Consequently, light is strongly reflected by a well-polished Diamond face.

In addition to its high refraction, the Diamond also exhibits considerable difference in refraction for the different colours of the spectrum, it has what is called a high dispersion Because of this, when white light is sent at an angle into a, Diamond, the Diamond acts in such a fashion as to produce brilliant prismatic colours. This is the celebrated 'fire' of the, Diamond which is seen to especial advantage either if the dia­mond is moved slightly or if the actual light flickers, such as candle-light for instance, which is incidentally by far the best kind of illumination to show off a brilliant gem.

A further consequence of the high refractive index is the ease with which light is reflected within the Diamond. Any beam inside the Diamond which strikes a face at an angle exceeding 24^ degrees is reflected internally and finds its way back to an observer who has a light source somewhere behind him. This is the whole key to the design of the brilliant gem. It is cut with faces set at carefully chosen angles such that when light enters the Diamond it finds a face which totally reflects it back. The upshot is that all the light falling on the gem comes back, indeed as if the gem were as bright a reflector as silver; but more so, for en route the dispersion operates so I hat a great deal of the white light impinging on the Diamond comes back in fiery spectral colours.

It is fairly clear that the purely optical characteristics of Diamond hardly come into consideration in connection with industrial uses, but there is one quite minor use of some scientific interest. This is in connection with transparency in the infra-red region of the spectrum. Most Diamond is opaque to ultra-violet light and also opaque to infra-red radiation, even if perfectly clear and transparent to visible light. There are, however, a certain number of Diamonds (classed as Type II lo distinguish them from the others which are classed as Type I) which are highly transparent in the infra-red. There is a good deal of scientific interest today in the infra-red, both from the viewpoint of industrial technology and also as a military strategic matter, for now there have been developed a number of what are called infra-red image converters.

These are devices which 'see' an infra-red invisible object, such as a hot gun-barrel, an exhaust from a vehicle, a hot missile and so on, and then electronically this invisible infra-red image is converted on a screen to a visible image. Clearly, the initial infra-red image detector must have a window which is highly transparent to a wide range of infra-red wavelengths. Actually, this is a considerable severe imposition technically, and it turns out that one of the best possible materials for such a window is a Type II Diamond.

Thus it is that this special (infra-red) optical property is of real value and application. l Strangely enough, Type II Diamonds are of a most unusual occurrence. They are relatively rare amongst Diamonds of suitable size, say 1 Carat upwards, and are certainly present to less than one in a thousand. Yet we ourselves have found that they are common amongst microdiamonds, even exceeding 20% of these! Such microdiamonds are not usable as infra­red windows. Stranger still, we are informed by an expert in Johannesburg that practically all the rare very large valuable gem-quality Diamonds (that is, those submitted to test) are in fact Type II also. There is naturally no question at all of cutting up such very rare large Diamonds for windows; they are far too valuable as gemstones for this scientific application even to merit the remotest consideration.