Spectroscopy begins with a simple observation we can all relate to:
However, there is much more to a rainbow than meets the eye, literally. Human eyes can only see a tiny slice of the full color spectrum that make up rainbows. This was discovered due to heat.
In 1800 William Herschel noticed that light passing through colored filters would transfer different amounts of heat. This lead to a fascinating experiment designed to determine the energy of different colors by placing a thermometer across the colors of the spectrum and observing changes in temperature.
The experiment led to an unexpected discovery. Herschel noticed that the highest temperature was beyond red where there was no visible light. He went on to find that these “heat rays” could be transmitted, reflected and absorbed just like visible light. This portion of the spectrum is known as infrared (“below red”) radiation.
One year later Johann Ritter discovered more invisible light on the violet side of the rainbow. He found that that photographic paper exposed to the rainbow will darken towards violet light (which is why we use red lightbulbs when working with unexposed film). When he put the paper beyond the visible violet light it quickly turned black due to ultraviolet (UV) radiation. UV radiation is responsible for the burns we experience when skin is exposed to direct sunlight.
Image Credit: NASA
These simple discoveries were only the tip of the iceberg. Over the next 130 years investigators made a series of discoveries leading to the modern understanding of the electromagnetic spectrum. You can think of this spectrum as the “full rainbow” as compared with the smaller spectrum visible to our naked eyes.