Infrared and ultraviolet radiation
If a spectrum from an electric arc lamp or from the sun is produced on a screen by one of the methods described in this chapter it may be shown that invisible radiant energy is incident on the screen just beyond each of the extreme ends of the visible spectrum (Fig. 25.7).
Just beyond the red end of the spectrum is a region occupied by infrared radiation. This is invisible, but its presence may be demonstrated by placing a thermopile where the rays can fall on it. A galvanometer connected to the thermopile will give a deflection. Details of the thermopile are given on page 192. Ultraviolet radiation can be detected in several ways. If a piece of photographic paper is placed on the screen it will become darkened to a variable extent where the spectrum falls on it, but maximum darkening is caused by ultraviolet radiation just beyond the violet end. Ultraviolet radiation also causes certain substances to fluoresce, i.e., glow with visible light. Quinine sulphate is an example. A colourless solution of this salt fluoresces with a blue light under the action of ultraviolet
radiation. Paper lightly smeared with vaseline behaves similarly. Laundry washing powders usually contain small quantities of a substance which fluoresces with a bluish white light under the action of the ultraviolet radiation in sunlight. This enhances the whiteness of linen and helps to combat the natural darkening of the material with age. More is said about the radiant energy spectrum on page 305.