The electromagnetic – or EM – spectrum is made up of seven kinds of electromagnetic energy with each corresponding to a different range. From lowest energy to highest energy, the seven groupings along the spectrum are: Radio Waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-rays, and Gamma Rays. Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. Read in the order listed above, waves increase in frequency and decrease in wavelength. The visible range, which is the only portion of the spectrum the human eye can detect, represents only a very tiny portion of the entire electromagnetic spectrum.
All electromagnetic radiation is made up of up of fields of electricity and magnetism interacting with each other. Electric fields can be static – like the static electricity that can hold a balloon to the wall. Magnetic fields can be static, too – like what holds a refrigerator magnet in place. However, electric and magnetic fields can also change and move together, and when that happens, the interaction produces waves: electromagnetic waves. EM energy can be described by frequency, wavelength, or energy, all of which are inter-related by the expression E = Frequency / Wavelength. Frequency is directly proportional to energy (they increase and decrease together) while wavelength is inversely proportional to energy (as wavelength increases, energy decreases).
Radio and microwaves are usually described by frequency (units of Hertz), infrared and visible light by wavelength (units of meters), and x-rays and gamma rays by energy (units of electron-volts). Though referred to by different names – light, EM radiation, or rays – all EM energy is made up of the same kinds of waves. The convention of using different units for different parts of the spectrum is simply a convenience that has to do with using numbers that are neither too large nor too small. The distinctions between the energy bands are simply a convention that eases communication. The EM spectrum doesn’t actually have breaks or chapters.
When you think of a water wave in the ocean, it might be easy to imagine the water oscillating up and down, creating a traveling waveform across the water. Even easier to imagine: making waves travel along a jump-rope secured to a wall at one end. In that case, it’s easy to see that the wave’s oscillation is perpendicular to the direction of its forward movement. In other words, the movement of the rope may be up and down, but the wave that travels through the rope is moving forward or backward—two perpendicular directions. These kinds of waves are called “transverse” waves. In transverse waves, the direction of the wave is perpendicular to the direction of applied energy. Another type of wave is a “longitudinal” wave, in which the wave moves parallel to the applied energy. With sound waves and other longitudinal waves, molecules vibrate and bump into one another, passing energy along the same direction the wave is moving.
While some transverse waves and some longitudinal waves might be easy to imagine, electromagnetic wave are harder to visualize. Because the wave is traveling in a direction that’s perpendicular to both the electric field and the magnetic field, thinking about EM waves requires three-dimensional visualization. Transverse waves like the jump-rope example give a close approximation, but electromagnetic radiation is more complex. One important feature of EM radiation is that, since its movement is based on the interaction of electric and magnetic fields, and electric and magnetic forces are possible over long distances, EM waves can travel through a vacuum. No material medium is necessary. Remember that low-energy EM radiation has longer wavelengths, corresponding to lower frequencies. High-energy EM radiation has shorter wavelengths, corresponding to higher frequencies.
All EM radiation travels at the same speed: the speed of light.
The categories along the spectrum – Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma Ray - represent a useful breakdown of EM radiation that helps scientists understand and visualize energy sources on, in, and under the Earth as well as throughout our solar system, galaxy, and universe.
You probably know ultraviolet – or UV – rays as the causes of sunburns. The Sun emits a wide variety of EM radiation, including infrared (which we feel as heat), visible (which we see with our eyes), and ultraviolet. The ultraviolet EM band, which has shorter wavelengths (10 to 400 nm) than visible light, is generally divided into three categories: UV-A, UV-B, and UV-C, with C being the most energetic. UV-C and most UV-B waves are absorbed by the ozone in Earth’s atmosphere. So, the reason we use sunscreen is actually to block the lower energy UV-A. Energetic EM radiation – like UV, X-rays, and gamma rays - can be harmful to living cells. The high-energy waves deliver intense energy bursts that can disrupt their functions, such as DNA replication. On the other hand, low-energy UV radiation can be useful for the growth of green plants.
UV light plays an important role in our current understanding of climate change and environmental science. Our atmosphere’s ozone layer is a crucial buffer that protects life on Earth from harmful UV rays, as it absorbs much of the UV radiation emitted by the Sun. Because of factors that scientists attribute at least in part to human activity, the ozone layer has developed a hole – and every year, it gets bigger, allowing more harmful UV rays to reach Earth’s living environments.
Ultraviolet light was discovered in 1801 by Johann Ritter, who conducted an experiment to explore the energy of light beyond the visible band of colors. He used photographic paper as a test background, since it was known that blue light would act on the paper more quickly than red. When the region beyond the blue range of the paper turned black very quickly, it indicated the presence of ultraviolet light. Humans can’t see UV light, but some insects, like bumblebees, can. Our telescopes and sensors can detect UV radiation, and scientists use data from UV sources to study solar activity, the composition of the Moon, star formation, evolution of galaxies, and the early universe.
NARRATION: Swirling spiral arms of galaxy M33 can be seen in visible light; but, the true extent of these spiral arms are revealed in ultraviolet light. Just as a dog can hear a whistle just outside the range of human hearing, bugs can see light just outside the range our eyes can see. A bug zapper emits this ultraviolet light to attract insects.
Johann Ritter conducted an experiment in 1801 to find out what, if any, electromagnetic waves are beyond violet. Ritter knew that photographic paper would turn black more rapidly in blue light than in red light, so he tried exposing the paper beyond the violet end of the visible spectrum. Sure enough, the paper turned black, proving the existence of light beyond violet: ultraviolet rays.
These ultraviolet rays, or UV radiation, vary in wavelength from 400 nanometers to 10 nanometers and can be subdivided into 3 regions, UV-A, UV-B, and UV-C. Visible light from the Sun passes through the atmosphere and reaches the Earth’s surface. UV-A, long wave ultraviolet, is the closest to visible light. Most UV-A also reaches the surface, but shorter wavelengths, called UV-B, are the harmful rays that cause sunburn. Fortunately, about 95% of these harmful UV-B rays are absorbed by ozone in the Earth’s atmosphere. UV-C rays are the shortest and most harmful, and are almost completely absorbed by our atmosphere. The ozone monitoring instrument aboard NASA’s Aura satellite detects ultraviolet radiation to help scientists study and monitor the chemistry of our atmosphere, including UV absorbing ozone.
While atmospheric protection from harmful UV radiation is good for humans, it complicates the study of naturally produced UV rays in the universe by scientists here on the Earth’s surface. Young hot stars shine most of their light beyond the visible light spectrum at ultraviolet wavelengths. Scientists need telescopes above the Earth’s UV absorbing atmosphere to find and study these UV bright regions of star formation and distant galaxies. New young stars in the spiral arms of galaxy M81 can be seen in this Galaxy Evolution Explorer, GALEX, image from NASA. Chemical substances, both atoms and molecules, interact with UV light, making this region particularly interesting to scientists. An ultraviolet instrument aboard Cassini has detected hydrogen, oxygen, water ice, and methane in the Saturn system. UV data have also revealed details of Saturn’s aurorae. Scientists also use UV waves shining from distant stars to reveal permanently shadowed regions of lunar craters. The Lyman Alpha Mapping Project, or LAMP instrument, aboard NASA’s Lunar Reconnaissance Orbiter, can use this faint star shine to look for possible water ice on the Moon.
Ultraviolet rays maybe harmful to humans, but they are essential to studying the health of our planet’s protective atmosphere, and give us valuable clues to the formation and composition of distant celestial objects.
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