What is the electromagnetic spectrum?

Electromagnetic spectrum: Vertical stripes of color fading from red through yellow, green, blue to purple.
The electromagnetic spectrum is a wide range of light; most of which we can’t see with our eyes. Here are the colors that make up the visible part of the spectrum. Image via Shutterstock.

The electromagnetic spectrum

When you think of light, you probably think of what your eyes can see. However, the light our human eyes can detect is only a sliver of the total amount of light that’s out there. The electromagnetic spectrum is the term scientists use to describe the entire range of light that exists. From radio waves to gamma rays, most of the light in the universe is, in fact, invisible to us.

Light is a wave of alternating electric and magnetic fields. The propagation of light isn’t much different than waves crossing an ocean. Like any other wave, light has a few fundamental properties that describe it. For example, one is its frequency, measured in hertz/ (Hz), which counts the number of waves that pass by a point in one second. Another closely related property is its wavelength: the distance from the peak of one wave to the peak of the next. In fact, these two attributes are inversely related. The larger the frequency, the smaller the wavelength, and vice versa.

Line of colors fading from red to violet with ROY G BV above them.
You can remember the order of the colors in the visible spectrum with the mnemonic ROY G BV. Image via University of Tennessee.

Our eyes see visible light

The electromagnetic waves your eyes detect – visible light – oscillate between 400 and 790 terahertz (THz). To put it another way, that’s several hundred trillion times a second. As an illustration, the wavelengths are roughly the size of a large virus: 390 – 750 nanometers (1 nanometer = 1 billionth of a meter; a meter is about 39 inches long). Our brain interprets the various wavelengths of light as different colors. For example, red has the longest wavelength, and violet the shortest. When we pass sunlight through a prism, we see that it’s actually composed of many wavelengths of light. The prism creates a rainbow by redirecting each wavelength out at a slightly different angle.

Diagram of spectrun showing scale of wavelengths from radio to gamma rays.
The entire electromagnetic spectrum is much more than just visible light. It encompasses a range of wavelengths of energy that our human eyes can’t see. Image via Wikimedia Commons.

But light doesn’t stop at red or violet. Indeed, just like there are sounds we can’t hear, there is an enormous range of light that our eyes can’t detect. In general, the longer wavelengths come from the coolest and darkest regions of space. Meanwhile, the shorter wavelengths measure extremely energetic phenomena.

The coolest part of the electromagnetic spectrum

Astronomers use the entire electromagnetic spectrum to observe a variety of things. Radio waves and microwaves are the longest wavelengths and lowest energies of light. With this in mind, they are used to peer inside dense interstellar clouds and track the motion of cold, dark gas. Radio telescopes have been used to map the structure of our galaxy. Additionally, microwave telescopes are sensitive to the remnant glow of the Big Bang.

Large wispy oval, red on one end, blue on the other.
This image from the Very Large Baseline Array (VLBA) shows what the galaxy M33 would look like if you could see in radio waves. This image maps atomic hydrogen gas in the galaxy. The different colors map velocities in the gas: red shows gas moving away from us, blue is moving towards us. Image via NRAO/ AUI.

Infrared telescopes excel at finding cool, dim stars, slicing through interstellar dust bands. Plus they even measure the temperatures of planets in other solar systems. The wavelengths of infrared light are long enough to navigate through clouds that would otherwise block our view. By using large infrared telescopes, astronomers peer through the dust lanes of our galaxy into the Milky Way’s core.

Dense starfield with bright patches, streaks and blobs.
This image from the Hubble and Spitzer space telescopes shows the central 300 light-years of our Milky Way galaxy, as we would see it if our eyes could see infrared energy. The image reveals massive star clusters and swirling gas clouds. Image via NASA/ ESA/ JPL/ Q.D. Wang/ S. Stolovy.

Most stars emit visible light

The majority of stars emit most of their electromagnetic energy as visible light, the tiny portion of the spectrum to which our eyes are sensitive. Because wavelength correlates with energy, the color of a star tells us how hot it is: red stars are coolest, blue are hottest. The coldest of stars emit hardly any visible light at all; they can only be seen with infrared telescopes.

The more energetic ultraviolet light

At wavelengths shorter than violet, we find the ultraviolet, or UV, light. You may be familiar with UV from its ability to give you a sunburn. Astronomers use it to hunt out the most energetic of stars and identify regions of star birth. When viewing distant galaxies with UV telescopes, most of the stars and gas disappear, and all the stellar nurseries pop into view.

Oblique spiral with yellow center and arms made of thousands of shining pale blue dots.
A view of the spiral galaxy M81 in the ultraviolet, made possible by the GALEX space observatory. The bright regions show stellar nurseries in the spiral arms. Image via NASA.

Highest energy light: X-ray and Gamma Ray

Beyond UV come the highest energies in the electromagnetic spectrum: X-rays and gamma rays. Our atmosphere blocks this light, so astronomers must rely on telescopes in space to see the X-ray and gamma ray universe. X-rays come from exotic neutron stars, the vortex of superheated material spiraling around a black hole. Or from diffuse clouds of gas in galactic clusters that are heated to many millions of degrees.

Meanwhile, gamma rays – the shortest wavelength of light and deadly to humans – unveil violent events. These include supernova explosions, cosmic radioactive decay, and even the destruction of antimatter. Gamma ray bursts are among the most energetic singular events in the universe. They are a brief flickering of gamma ray light from distant galaxies when a star explodes and creates a black hole.

Hand-shaped blue gas cloud with crown of yellow-orange spots above finger end.
If you could see in X-rays, over long distances, you’d see this view of the nebula surrounding pulsar PSR B1509-58. This image is from the Chandra X-ray Observatory.  Located 17,000 light-years away, the pulsar is the rapidly spinning remnant of a stellar core left behind after a supernova. Image via NASA.

Bottom line: The electromagnetic spectrum describes all the wavelengths of light, both seen and unseen.

July 8, 2022

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