Infrared radiation is a form of energy that exists as part of the electromagnetic spectrum, including visible light, radio waves, and X-rays. Unlike the light we perceive with our eyes, infrared radiation possesses wavelengths longer than those of red light, rendering it invisible to human vision. Despite its invisibility, this type of radiation is commonly experienced as heat, like the warmth from sunlight or a hot object. It occupies the region of the spectrum between visible light and microwaves, extending from approximately 780 nanometers to 1 millimeter.
The Unveiling of Infrared
The existence of infrared radiation was brought to light by Sir William Herschel, a German-born British astronomer. His groundbreaking discovery occurred in the year 1800, marking a significant moment in the understanding of light and energy. Herschel, who had previously achieved renown for discovering the planet Uranus in 1781, was a prominent figure in science during his time. It was during his investigations into the properties of sunlight that he stumbled upon this previously unknown form of radiation.
Herschel’s Groundbreaking Experiment
Herschel’s discovery stemmed from an experiment designed to investigate how much heat was carried by different colors of sunlight. He used a glass prism to split sunlight into its constituent colors, creating a spectrum similar to a rainbow. To measure the temperature of each color, he placed thermometers with blackened bulbs in various parts of this visible spectrum. Blackened bulbs absorb heat effectively. As he moved the thermometer from the violet end of the spectrum towards the red, he observed a gradual increase in temperature.
Herschel decided to place a thermometer just beyond the red end of the visible spectrum, in an area where no light was visible to his eyes. The thermometer in this invisible region registered the highest temperature. This unexpected rise in temperature indicated the presence of an unseen form of radiant energy, which he initially termed “calorific rays.”
Legacy of the Discovery
Herschel’s identification of infrared radiation expanded humanity’s understanding of the electromagnetic spectrum. Before his work, visible light was largely considered the extent of light itself. His discovery demonstrated that there were forms of radiation beyond what the human eye could perceive, yet these forms still exhibited properties similar to visible light, such as reflection and refraction.
This revelation laid a foundation for the eventual mapping of the electromagnetic spectrum, inspiring further investigations into other invisible forms of energy. Within a year of Herschel’s finding, Johann Wilhelm Ritter identified ultraviolet light beyond the violet end of the spectrum. Herschel’s experimental approach and the recognition of an invisible form of light were significant, shaping modern physics and astronomy.