See just how narrow the visible light band is relative to other EM energy – and why, despite that, it’s crucially important to humans. Explore the relationship between color and temperature, the appearance of the atmosphere (and why the sky is blue!), and how scientists use light scattering to figure out what things are made of - both on and off Earth - in this video from NASA.
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.
Visible light, though crucial for humans, is just a tiny sliver of the entire EM spectrum. From radio waves to gamma rays, the spectrum spans 28 orders of magnitude – from wavelengths of 10-21 meters to 107 meters. Visible light, at wavelengths of approximately 10-7meters, is just a very small segment of the total spectrum.
All electromagnetic radiation is considered to be light. Sometimes when we refer to “light” we mean just visible light, but when scientists refer to light, they usually mean all types of EM radiation. (Sometimes the visible band of the EM spectrum is referred to as the “optical” band.) Human eyes can’t see wavelengths that are larger or smaller than those in the visible band, covering wavelengths from about 400 (violet light) to 700 nanometers (red light). In 1665, Newton showed that what we perceive as white light is actually a spectrum of various colors. Using a prism, he broke up white light into what is now the familiar color spectrum. Each color has a different wavelength and thus refracted through the prism at a different angle.
As objects change temperature, they change color – meaning they radiate EM waves of different wavelengths, frequencies, and energies. Astronomers have used this relationship to categorize and classify stars by temperature and color. Our Sun is predominantly yellow based on its temperature. If it were hotter or cooler, its dominant color would change. The red giant star Betelgeuse in the constellation Orion is cooler than our Sun. Rigel, another star in Orion, appear blue to our eyes, indicating that it is much hotter than both Betelgeuse and our Sun. The visible color spectrum radiated by a star indicated not only temperature but also the presence of particular elements in its structure.
Any wavelength of light can be used for spectroscopy – the measure of energy absorption or emission by or from an observed object. Patterns of absorption or emission lines can tell scientists what elements or molecules are present in a sample. Visible light spectrographs of the Sun, planets, stars, and interstellar space help astronomers understand the origins and composition of the universe. Closer to home, scientists use orbiting satellites as “eyes in the skies” to capture optical images of land and sea data.
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