What is Infrared?

Infrared is a form of light... light that we can not see with our eyes, but that we can sometimes feel on our skin as heat.

When we think of light, we may imagine the glare of the Sun on a summer day, or the soft glow of a light bulb at night. But visible light, the only light our eyes can see, makes up just a tiny sliver of all the light in the world around us.

Infrared light falls just outside the visible spectrum, beyond the edge of what we can see as red. Sir William Herschel first discovered infrared light in 1800. He split light into a rainbow (called a spectrum) by passing sunlight through a prism, and then placed a thermometer in different colors in that spectrum. Unexpectedly, he found the thermometer showed a rise in temperature, even when placed in the dark area beyond the edge of the red light. He hypothesized that there must be more light beyond the color red that we simply could not see with our own eyes. You can recreate Herschel's experiment yourself with a box, a prism, three thermometers, and a few other common supplies. 

Light does not stop at just the visible and infrared. Other types of light that you may have heard of include gamma rays, X-rays, ultraviolet, microwave, and radio. Everything in this spectrum travels at the ultimate speed limit of the Universe which is, of course, the speed of light.

The Electromagnetic Spectrum

Light travels through the Universe as a wave, but it is rather different than the ripples we see moving across the surface of a lake. Light waves are made up of electric and magnetic fields. So another name for light is electromagnetic radiation. And the entire spectrum of light is similarly called the electromagnetic spectrum.

One of the basic properties of any wave is its wavelength, which is just the distance between the peaks of one ripple, or wave, and the next. For light, it is the length of one full cycle, or pulse, of the electric and magnetic fields. A related property is the frequency, or the number of waves that pass a fixed point every second.

Our eyes detect differences in the wavelength of visible light as differences in color. Essentially, color is your brain's way of converting the different wavelengths of light that your eyes see into something that you can quickly understand. Red light has a longer wavelength than green light, which in turn has a longer wavelength than blue light. The wavelength of infrared light is longer than red light, in some cases many hundreds of times longer. These longer wavelengths carry less energy than red light and do not activate the photoreceptors in our eyes, so we cannot see them.

Heat and Light

Since we think of infrared light as something that makes us feel warm, is there a connection between heat and light? Are they the same thing?

The real connection is that everything in the Universe that is warm also gives off light. This is true of stars, planets, people, and even the Universe itself! Physicists call this light blackbody radiation. Every object in the Universe, even one that is as black as a lump of charcoal, will give off this light. Where this light falls in the spectrum, however, depends on the temperature of the object.

Cooler objects glow faintly at longer wavelengths of light, while hotter objects glow more brightly at shorter wavelengths. Our Sun's temperature is a blistering 5,778 K (9,940° F), which is so hot that it glows brightest at visible wavelengths of light (around 0.4 - 0.7 microns). People, who are much cooler (310 K, 98° F), actually glow as well, but in infrared light with a wavelength of around 10 microns. A micron is a millionth of a meter. 

The Cool Cosmos

Astronomers who want to study the most frigid things in the Universe turn to infrared telescopes to reveal their faint glow. Clouds of dust that range from hundreds to tens of degrees above absolute zero appear as black soot in visible light, but glow brightly at infrared wavelengths out to several hundred microns.

So, to study the cool cosmos, infrared light is our window into the heat of the coolest things around.