Sir William Herschel discovered infrared light in 1800. This breakthrough came during an experiment where he measured heat from different colors of sunlight passed through a prism.

The Serendipitous Experiment

Herschel, a German-born British astronomer famous for discovering Uranus in 1781, was curious if colored filters affected heat from sunlight. He split sunlight into a spectrum using a glass prism and placed thermometers in each color—violet, blue, green, yellow, orange, and red. Temperatures rose from blue to red, but the highest reading came just beyond the red edge, where no visible light appeared. This "calorific ray," as he called it, proved invisible light existed, which we now know as infrared radiation. He published his findings in Philosophical Transactions of the Royal Society in April 1800.

"I found it proved beyond a doubt that the full red falls still short of the maximum of heat... There exists, therefore, a bright light-like ray beyond the red." – William Herschel, 1800

How He Did It: Step-by-Step

Herschel's setup was brilliantly simple yet revolutionary:

  1. Prism Setup : Direct sunlight through a glass prism onto a dark surface to create a rainbow spectrum.
  2. Thermometer Placement : Use mercury thermometers with blackened bulbs (to better absorb heat) in each color band and one as a control outside the spectrum.
  3. Measurements : After minutes of exposure, record temperatures—blue around ambient, red hotter, and beyond-red hottest (often 5–10°C higher).
  4. Confirmation : Repeat with different prisms and conditions to rule out errors.

You can recreate this at home safely with sunlight, a prism, and an infrared thermometer for modern precision.

Historical Context and Impact

In early 1800, amid the scientific fervor post-French Revolution, Herschel's work expanded the known electromagnetic spectrum just after visible light's boundaries were probed. His sister Caroline assisted in his observatory, though this infrared discovery was solo. It paved the way for infrared astronomy, night vision, remote controls (using ~940 nm IR LEDs), medical imaging like Face ID, and even air fryers. By February 2026, infrared tech powers cutting-edge uses like self-driving car sensors and climate monitoring satellites.

Aspect| Visible Light| Infrared Light
---|---|---
Wavelength| 400–700 nm| 700 nm–1 mm 3
Detection| Human eyes| Thermometers, sensors
Herschel's Finding| Expected heat gradient| Unexpected max beyond red 1
Modern Apps| Cameras, displays| Remotes, heat cams, therapy 2

Multiple Perspectives and Trivia

  • Astronomer's View : Herschel saw it as "heat rays" from stars, boosting his telescopes' legacy.
  • Physics Angle : Confirmed light's wave nature, predating Maxwell's equations.
  • Fun Fact : No recent "rediscovery" trends in 2026 forums, but viral recreations pop up yearly on YouTube—timeless science!
  • Debate Note : Some credit precursors like Johann Ritter (UV in 1801), but Herschel's IR is undisputed first.

TL;DR: William Herschel found infrared in 1800 by spotting max heat beyond red light in a prism experiment—sparking invisible light tech we use daily.

Information gathered from public forums or data available on the internet and portrayed here.