From the March 2015 issue

Beyond the red

Exploring the warm, cozy weirdness of infrared light.
By | Published: March 30, 2015
Invisible light? No one saw that coming. It was as astounding in 1800 as the discovery of microbes some 125 years earlier.

This story begins with the Sun, which radiates about 40 percent of its energy as the familiar spectral colors. The other 60 percent is invisible, and most of that is infrared (IR), which our skin perceives as heat. IR is just like red light, except its waves are slightly more spread out. Each wave has about half the thickness of a human hair.

Late in the American Revolution, William Herschel gave Britain a badly needed boost to its pride when he found the first-ever new planet, Uranus. He was also an accomplished symphonic organist, cellist, and oboist as well as the composer of 24 symphonies, even if each was renowned for curing insomnia. An expert lens maker, he built the largest telescopes of his day, including a star-party colossus with a 48-inch mirror. He discovered two moons of Saturn and two orbiting Uranus, and coined the word asteroid. Herschel was a celebrity.

Nineteen years after his Uranus discovery, Herschel was fooling around with various filters for solar viewing when he noticed that red light seemed to be accompanied by more heat than any other color. Intrigued, he placed a thermometer on a table where cut glass cast the colorful solar spectrum. In one of those eureka moments most of us only get when we suddenly remember we can mute a commercial, he wondered if the red section was hotter than the green or blue. It was a simple idea, but nobody had ever thought of it. What he found dropped his jaw. The mercury rose the most when the thermometer was held not in the visible spectrum, but next to it. “Invisible light” must be hitting that dark spot on the table near the red. Herschel called these “calorific rays.”

Today we know that infrared, the modern name, has waves of the optimum size to jostle whole atoms and molecules. When atoms move, we call it heat. So infrared is not heat, but it does produce it. And it works both ways: Jiggling atoms create infrared light. It’s a spicy symbiotic dance.

Humans can’t directly sense infrared. Rather, we feel the speed-up of our skin’s atoms caused by IR. Nonetheless, “heat” and “infrared” are popularly used interchangeably, as in “heat lamp.”


Infrared vibrates at the same frequency that glass molecules like to rattle. Glass and IR resonate violently, creating a chaotic barrier. So while visible light easily penetrates windows, infrared does not. After light comes through, the heat it creates cannot get back out. Glass creates an infrared trap! Like your car in sunlight.

Infrared also does not like to scatter. Its long waves are penetrating. We see this principle, called Rayleigh scattering, in everyday life. Distant mountains appear blue because sunlight’s shorter waves bounce among the air molecules between us and the mountains to create a bluish haze. Green scatters much less, and red light less still.

If you observe life through rose-colored glasses (or even yellow-tinted ones), which block blue but let red pass through, you see those far-off mountains illuminated mostly in sunlight’s red component. They stand out sharply. It’s dramatic. The haze is gone because you no longer see the scattered-around blue. This is the principle behind those amber “blue-blocking” sunglasses hawked on late-night TV, which you’d certainly buy if you hadn’t foolishly muted the commercial.

If red doesn’t scatter much, infrared scatters even less. Exploiting this, astronomers can detect IR from the center of our galaxy, whereas visible light has been scattered and blocked by all the dusty nebulae so that we cannot see a thing. No telescope can see the Milky Way’s core, 27,200 light-years away, hidden behind Beijing-level pollution. But IR telescopes essentially feel its heat and create structural maps of what’s there.

Namely, lots of stars. All create copious infrared. Including ours, of course. The high solar IR flux reveals that the Sun is hot. And you bought a magazine to learn this.

Although half the Sun’s energy is infrared, a much greater IR percentage generally floods the cosmos. That’s because the expanding universe shoves most galaxies’ visible light into the IR. For astronomers, the widespread redshift has made the IR realm the fashionable place to be. Nowadays, many major observatories — including the under-construction James Webb Space Telescope — obsessively focus on the infrared. It’s also the only part of the spectrum amenable to adaptive optics, although recent advances have begun to extend that image-steadying technique into the deep red.

Spring’s ever-higher Sun now cranks up its infrared, warming our winter-weary skin. It’s nice to know it’s also unlocking the universe’s secrets.

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