Return to Scale View
About

How big is the Wavelength of Infrared Light?

15 מיקרומטרים!

View History Report
Infrared Wavelength

Infrared light is light with waves too long for human eyes to see. The explorer shows an infrared wave 15 micrometers long. A micrometer is a millionth of a meter, or a thousandth of a millimeter. Waves of this length are among those that Earth gives off as heat.

Wavelength

Light travels as a wave, and its wavelength is the distance from one crest of the wave to the next. Infrared waves are longer than the waves of visible light. People cannot see them, but they can feel some of them as heat.

The International Commission on Illumination (CIE) puts the infrared range at about 780 nanometers to 1 millimeter. A nanometer is a thousandth of a micrometer. The longest infrared waves are about 1,300 times as long as the shortest. The CIE divides the range into IR-A (780–1,400 nanometers), IR-B (1.4–3 micrometers) and IR-C (3 micrometers to 1 millimeter), so a 15-micrometer wave is IR-C.

Other fields speak of near, mid and far infrared, and the CIE notes that the borders between them vary with the application. NASA's James Webb Space Telescope counts 4.9–28.8 micrometers as mid-infrared. Earth scientists call 8–15 micrometers the thermal infrared, because those wavelengths are best for studying the heat that Earth gives off. The explorer's 15 micrometers is at the long edge of that band.

Size comparisons

  • About five to seven 15-micrometer waves, laid end to end, would span the width of a human hair, which is 80–100 micrometers wide.
  • A 15-micrometer wave is about 20 times as long as a wave of red light, the longest that people can see, at about 700 nanometers.
  • A TV remote control uses infrared light about 940 nanometers long. A 15-micrometer wave is 16 times as long.
  • One 15-micrometer wave is 1.5 times the width of the 10-micrometer white blood cell next to it in the explorer, and about twice the width of a red blood cell, which is 7–8 micrometers across.

Infrared light and heat

William Herschel discovered infrared light in 1800. He placed thermometers in each color of sunlight and found that the temperature rose from blue to red. Beyond the red end, where he could see no light, it was warmer still.

Very hot objects, such as a fire, give off visible light as well as infrared. Cooler objects, such as people and animals, give off only infrared. Infrared cameras and night-vision goggles detect that light, so they can show warm bodies that eyes cannot see.

Earth's heat and carbon dioxide

Earth loses heat to space as infrared light. The European Space Agency reports that more than 97 percent of the energy Earth gives off has wavelengths between 4 and 100 micrometers. Gases in the air absorb some of it and warm the surface, which is called the greenhouse effect. It keeps Earth's average surface temperature at about 15 degrees Celsius, more than 30 degrees warmer than it would be without an atmosphere.

Carbon dioxide strongly absorbs infrared light longer than about 12–13 micrometers, including 15 micrometers. That is close to where Earth's surface gives off the most energy, at around 12.5 micrometers. Water vapor absorbs little of the light between about 8 and 14 micrometers, an atmospheric window through which much of it escapes to space, and more carbon dioxide partly closes that window.

Seeing space at 15 micrometers

The Webb telescope's Mid-Infrared Instrument, called MIRI, can take pictures at 15 micrometers. Its detectors must be kept below 7 kelvins, 7 degrees above absolute zero, the coldest possible temperature. The telescope carries a refrigerator called a cryocooler for them alone.

In 2023, astronomers used MIRI to take the temperature of TRAPPIST-1 b, a rocky planet about 40 light-years from Earth. A light-year is the distance light travels in a year. The planet's dayside glowed at 15 micrometers as brightly as a surface of about 500 kelvins (about 230 degrees Celsius).

That matches bare rock with no atmosphere to spread the heat around. An atmosphere with a lot of carbon dioxide would have given off less 15-micrometer light and made the planet look cooler.

Sources

סדרי גודל ביקום > Infrared Wavelength
White Blood Cell
These mass murderers are also known as leukocytes. There are many different types of this disease-fighting cell. The most common type, the neutrophil, has multiple nuclei. The largest type, the macrophage, has a diameter of a whopping 21 µm.
Mist Droplet
Mist can reduce visibility a bit. It can also reflect light, so that you can see rays of light shining through it. That's pretty cool. If you want to find mist, go to the coast or the mountains. Mist is more common there.
Red Blood Cell
Around one quarter of all human cells are red blood cells. They circulate through one cycle every 20 seconds, and only live around four months. The dent allows for more surface area, which means it's easier to transport oxygen. Oh, and they have no nucleus.
Twip
A twip is a twentieth of a typographical point. That means that 12-point font is approximately 240 twips high. Also, TUIP can stand for "The Universe in Perspective”, an old name for the Scale of the Universe. TUIP and twip are pronounced the same.
Thou
Although the word "thou" is most commonly known as being an old form of the word "you", a thou is also a unit of measurement. It is equal to one thousandth of an inch; in fact, that's where it gets its name. THOUsandth. Pretty cool.
Skin Cell
The outermost layer of skin, which is also the only visible layer of skin, is made up of dead cells. That's because dead cells are the strongest, and stronger skin provides more protection! These cells continuously fall off. In fact, dust is 95% dead skin cells!

Discussion