Invisible Light
Around 1800 while Young was developing his wave theory three scientists discovered that
the color spectrum was bordered by invisible rays. Sir William Herschel, a British
astronomer, was measuring the temperature of the colors dispersed by a prism. As he moved
the thermometer down the spectrum from violet to red, he observed a rise in temperature.
As he moved the thermometer beyond the red beam, the temperature grew even higher.
Herschel had discovered a hot, invisible radiation that appeared to be a continuation of
the spectrum. This radiation is called infrared radiation because it occurs just below red
in the spectrum, where there is no visible light.
Ultraviolet rays were discovered by Johann Wilhelm Ritter and by William Hyde Wollaston,
who were independently studying the effects of light on silver chloride. Silver chloride
placed in violet light grew dark. When the chemical was placed in the area beyond the
violet of the spectrum, it darkened even more rapidly. They concluded that a chemically
powerful kind of invisible radiation lay beyond the violet end of the spectrum.
In 1864 James Clerk Maxwell, a Scottish physicist, published a theory of
electricity and magnetism. He had developed equations that predicted the existence of
electromagnetic waves caused by electrical disturbances. He calculated the speed of such
waves and found it to be the same as the speed of light. Maxwell concluded that light was
an electromagnetic wave. As a single light wave travels through space, its movement
consists of the growth and collapse of electrical and magnetic fields. The electrical
fields are at right angles to the magnetic fields, and both are at right angles to the
direction in which the wave is moving.
Maxwell's theory implied that other electromagnetic radiations with wavelengths longer
than infrared or shorter than ultraviolet might be found. In 1887 Heinrich Hertz produced
radio waves, which have longer wavelengths than infrared rays, thus confirming Maxwell's
theory.
