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