Infrared Light: Beyond the Red

Dear Tom,
I have heard infrared light referred to as "beyond the red." What does this
mean?
Dale Halvers
Dear Dale,
The answer can be found in the year 1800. British astronomer Sir William
Herschel (1738-1822), while investigating the nature of light, moved a
thermometer through the spectrum of colors obtained by passing light through
a prism. Herschel observed that the thermometer registered higher
temperatures as it absorbed heat energy contained in the light. That outcome
was expected.
Much to Herschel's surprise, however, the temperature continued to rise as
he moved the thermometer "beyond the red" portion of the spectrum and into
the darkness. Herschel had discovered a previously unknown form of radiant
energy: invisible infrared light whose wave length lies just "beyond the
red" wavelength.