The fathers of spectroscopy, a series: Sir William Herschel

The fathers of spectroscopy, a series, Sir William Herschel

Spectroscopy is the study of the absorption, reflectance, and emission of electromagnetic radiation (e.g., light) by matter, measured as a function of wavelength (or frequency). It is used as a tool for characterizing the structures of atoms and molecules by exploiting the fact that they absorb frequencies that are characteristic of their structure.

Readers, while this is a mouthful of physics to kick this series off, it is important to recognize that this foundational knowledge came from somewhere!

In this post we are going to start at the beginning… back to the early 19th century with Sir Frederick William Herschell, a musician and astronomer from Germany, who, in addition to being one of the fathers of spectroscopy, famously discovered Uranus with the telescopes that he and his sister built.

In 1800, Herschel questioned how much heat is passed through different colored filters he used to observe sunlight. To test his hypothesis that different colors seem to pass differing amounts of heat (and therefore the colors themselves may be of varying temperatures!), he directed sunlight through a glass prism and created a spectrum, or the rainbow created when light is divided into colors.

Herschell subsequently measured the temperature of each color. As part of his experimental design, he used three thermometers with blackened bulbs (to better absorb the heat) and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as control samples. As he measured the individual light temperatures (i.e., violet, blue, green, yellow, orange, and red), he noticed that all the colors had temperatures higher than the controls. Moreover, he found that the temperatures of the colors increased from the violet to the red part of the spectrum.

…. now it’s getting interesting, right… BUT… how back in the 19th century did he measure ‘light’ that he could not see?…

After noticing this pattern of increasing temperatures, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all!

Herschel performed additional experiments on what he called calorific (calor is Latin for ‘heat’) rays beyond the red portion of the spectrum. He found that they were reflected, refracted, absorbed, and transmitted in a manner similar to visible light. This discovery is a form of light, or radiation, that is now known as the infrared.

This was the first time that someone demonstrated that are types of light that we cannot see with our eyes.

_This series will be continued with James Clerk Maxwell… _