As we all know, the sun emits an enormous amount of electromagnetic radiation. Of this, a very small portion lays within the visible spectrum — colors we can see, like red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). The sun also emits radiation outside of this band, but we didn’t always know that…
Sir William Herschel
Although Sir William Herschel was quite the Renaissance Man back in his day, his life didn’t start out that way. Through hard work and determination, Sir William made a name for himself as a musician and composer. At one fortuitous point in his life, he’d also started developing a fascination with astronomy.
Given the lack of scientific instrumentation in the late 1700’s, Herschel resorted to making his own scientific instruments including hand-ground mirrors and hand-made telescopes. It was in this pursuit of astronomy that led him to look into the topic of color — specifically, developing the best telescope filter that would let him look at the Sun through his telescope. However, it is not until we look at the timeline of his publications that things become really interesting. For instance, in 1794 he points out the heat coming from the Sun:
Among the celestial bodies the sun is certainly the first which should attract our notice. It is a fountain of light that illuminates the world! it is the cause of that heat which maintains the productive power of nature, and makes the earth a fit habitation for man!Herschel William. III. On the nature and construction of the sun and fixed stars. Phil. Trans. R. Soc. 8 546–72
In this publication, Herschel unwittingly mentions optical wavelengths, heat, and the Sun. At this point, it doesn’t appear that he’s put the three concepts together. It isn’t until his paper to the Royal Society (read on April 24, 1800) that he makes this connection.
In the paper, Herschel discloses the results of an experiment he’s completed. In the experiment (illustrated below), Herschel uses a prism to split light from the sun into its spectral components. In his diagram, he labels seven different colors: violet, indigo, blue, green, yellow, orange, and red. Significantly, he then places thermometers beyond the red edge of the spectrum:
The thermometers being perfectly settled at the temperature of the room, I placed the stand so that part of the colour, refracted by the prism, fell on the edge of the paper, before the thermometer No. 1, and about half way, or 1 1/4 inch, towards the second
Here the thermometer No. 1 rose 6 1/2 degrees, in 10 minutes, when its centre was placed 1/2 inch beyond the visible light.Herschel William. 1800. XIV. Experiments on the refrangibility of the invisible rays of the sun. Phil. Trans. R. Soc. 90: 284–292
Without a doubt, it is this experiment that allows Herschel to tie together the Sun’s heat as a form of light, as well as its spectral location relative to visible light. The nail in the logical coffin comes near the end of his paper:
The first four experiments prove, that there are rays coming from the sun, which are less refrangible than an of those that affect the sight. They are invested with a high power of heating bodies, but with none of illuminating objects; and this explains the reason why they have hitherto escaped unnoticed.
To conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired, whether light be essentially different from radiant heat? In answer to which I would suggest, that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one.Herschel William. 1800. XIV. Experiments on the refrangibility of the invisible rays of the sun. Phil. Trans. R. Soc. 90: 284–292