In March 1800, a musician turned astronomer placed three thermometers on a table and aimed a sunbeam at them, passed through a prism. He wasn't searching for new radiation—he was measuring heat. But one of the thermometers, lying in darkness beyond the red edge of the spectrum, showed a higher temperature than all the others.
William Herschel came to England from Hanover in 1757—an oboist in a military band, fleeing from war. For twenty years he earned his living through music: performing, conducting, composing concerts. Telescopes entered his life as a hobby—he ground mirrors at night, between rehearsals. His sister Caroline polished the metal while her brother conducted observations. On March 13, 1781, Herschel saw through the eyepiece a pale spot moving among the stars. The first planet discovered since antiquity. Uranus.
King George III granted Herschel a lifetime stipend—200 pounds per year. The musician became court astronomer. He built a telescope with a mirror 122 centimeters in diameter—the world's largest at that time. But his next discovery happened not at the eyepiece, but at a table with a prism.
Herschel knew that different colors of the spectrum heated objects differently. Red filters transmitted more heat, violet ones less. He wanted to understand whether one could observe the Sun through colored glass without risking one's eyes. To do this, he needed to measure how much heat each section of the spectrum carried.
The experiment looked simple. A sunbeam passed through a slit in the shutter, fell on a glass prism, decomposed into color bands. Herschel placed the central thermometer in a color band—first in red, then in green, then in violet. The other two lay to the sides, in shadow outside the spectrum, as controls. He recorded the temperature every few minutes.
The results weren't unexpected. The red section heated the thermometer by 6.9°F, green by 3.2°F, violet by 2°F. Heat decreased from the red edge to the violet, as anticipated. But Herschel noticed that the control thermometer lying in shadow next to the red edge was also heating up—though no visible light fell on it.
He moved the thermometer farther beyond the red band, to where the eye saw nothing. The temperature rose. Farther still—it rose again. The heating peak turned out to be not in the red, but beyond it, in a region Herschel called "invisible heat rays."
This wasn't a chance finding. Herschel conducted a series of control measurements: he checked whether the prism itself was heating the thermometer, whether distance from the glass mattered, whether the effect changed at different refraction angles. He established that the invisible radiation obeyed the same laws of refraction as visible light. It reflected from mirrors. It passed through some materials and was blocked by others. Essentially, it was light—just invisible.
In 1800, the nature of light remained a mystery. Newton considered light a stream of particles, Huygens—waves in the ether. The dispute wasn't settled. Herschel leaned toward the corpuscular model, but his discovery didn't depend on theoretical preferences. He described the phenomenon as it was: beyond the red edge of the spectrum exists radiation that heats a thermometer but doesn't affect the eye.
The word "infrared" appeared later. Herschel himself called his finding "calorific rays"—a reference to caloric theory, the weightless fluid of heat that in 1800 hadn't yet been definitively rejected. He didn't know that heat rays and visible light were parts of a single electromagnetic spectrum. This idea would be formulated by James Maxwell only in the 1860s, uniting optics and electricity into one theory.
Herschel measured temperature with mercury thermometers—the only available instruments of that time. The thermocouple, more sensitive to small temperature changes, would be invented by Thomas Seebeck only in 1821. The bolometer, capable of registering weak radiation from distant stars, would be created by Samuel Langley in 1878. Herschel worked with instruments that barely detected the effect, but it was enough to prove: the spectrum doesn't end where vision ends.
Herschel's discovery remained a scientific curiosity for nearly a century. Infrared radiation was studied in laboratories, but no applications were found for it—too weak, too diffuse, too difficult to register. The breakthrough came not in science, but in war.
1943. German engineers brought into serial production detectors made of lead sulfide—a semiconductor sensitive to infrared radiation: sensors that caught hot engines of planes and tanks coexisted with the first night vision devices. The Americans responded with their own developments. By war's end, infrared detectors learned to see warm objects in darkness: tanks, planes, people. After the war, the technology transitioned to civilian science. Astronomers first gained an instrument capable of looking into that region of the spectrum Herschel had discovered with a thermometer.
In the 1960s, infrared telescopes began detecting sources invisible in the optical range: cold dust clouds, protostellar disks, galactic nuclei hidden behind gas. In 1983, the IRAS satellite reached orbit—the first infrared observatory in space. It mapped the sky in four ranges, found more than 350 thousand sources, discovered dust disks around Vega and Fomalhaut—the first hints of planetary systems around other stars.
Spitzer, launched in 2003, saw light from the first galaxies born a few hundred million years after the Big Bang. James Webb, put into orbit in 2021, operates primarily in the infrared range—its 6.5-meter diameter mirror collects radiation from exoplanets, nebulae, quasars. It sees what's inaccessible to optical telescopes: objects so distant that their light has stretched into the infrared region due to the expansion of the Universe.
Herschel wasn't the only one to look beyond the edge of the spectrum. A year later, in 1801, German physicist Johann Ritter discovered that beyond the violet edge of the spectrum exists radiation that blackens silver chloride faster than visible light. He called it "chemical rays"—later they'd be called ultraviolet. Two discoveries, two edges of the spectrum, two regions of invisible light.
But Ritter is barely remembered. His experiment didn't get quick continuation—ultraviolet remained more a laboratory curiosity than a tool for a long time: only by the end of the 19th century, when it became clear that it caused chemical reactions in living tissues, did physicians take interest. Infrared radiation, by contrast, became a tool immediately after detectors appeared. Herschel gave science not just a new section of spectrum, but a new way of looking at the world—through heat instead of light.
His experiment took several days. Three thermometers, a prism, a sunbeam. But the result proved more momentous than the discovery of Uranus. A planet is an object you can see if you build a large enough telescope. Infrared radiation is an entire layer of reality, hidden from the eyes but accessible to measurement.
Today we see the Universe in ranges Herschel couldn't imagine: radio waves, X-rays, gamma rays. But the first to prove that the spectrum is wider than vision was a musician with a thermometer. And his discovery began not with a hypothesis, but with a simple question: why does a thermometer in shadow heat up more than one in light?