The Electromagnetic Spectrum
Even as a kid growing up in Southern California, I was always wondering: Why do things look the way they do? Why is the sky blue, but grass is green? I got into rock collecting and wondered at the variety of colors and textures – how did they form?
As I studied geology in college and grad school, I started to learn the answers and in my career at NASA’s Jet Propulsion Laboratory, I was able to apply that knowledge, using satellites to study the rocks and soils of Earth and other planets. I’ve also been able to pass some of that knowledge on to students in my classes at Santa Barbara City College, where I’ve been teaching a class on planetary exploration.
The science and technology we use to study the earth and planets from space can get pretty complicated, but it turns out you don’t have to be a rocket scientist to understand the basis for that technology. And it goes way beyond what we can see with our eyes, because we can use the electromagnetic spectrum in many other ways: it lets doctors see inside us, it warms us, cooks our food, helps us connect to the internet, and sends tunes to our car radios.
Isaac Newton was one of the first to notice that the “white” light we take for granted is instead made up of multiple colors. He used a prism – a triangular bit of glass – to bend a beam of light, which separated the colors because each component travels at a different speed through the glass.
Light turns out to be made up of waves and each color has its own wavelength. We now know that the light we see is just a small part of a much broader range of electromagnetic waves spanning from gamma and X-rays to ultraviolet through visible wavelengths to infrared and into radio waves. All share the property of waves, radiating out from a source, much like ripples on a pond after a pebble has been tossed. However, Einstein showed that electromagnetic waves could also be considered particles called photons, a consequence of quantum theory.
The spectrum includes a vast range of wavelengths, from gamma rays at 40 trillionths of an inch (1/1000th of a nanometer) to many feet (meters) in the radio spectrum. The visible part of the spectrum is rather small, ranging from violet at 10 millionths of an inch (400 nm) to red at 30 millionths of an inch (760 nm). It’s undoubtedly more than a coincidence that the sun’s spectral output spans the visible part of the spectrum; our eyes are optimized to see the wavelengths that are available.
Why is an Apple Red? Why is the Sky Blue?
Luckily, the small part of the spectrum we can see provides us with a lot of information about the world around us, through the colors we see. It all comes down to which wavelengths are reflected from an object and which are absorbed. For example, an apple is red because all the colors besides red (e.g. blue, green, yellow, orange) are absorbed and red is reflected to our eye. Likewise, the sky is blue because gases and dust in the atmosphere absorb and scatter the longer wavelengths, leaving blue.
The atmosphere does a lot more for us, because its gases, mostly ozone, absorb ultraviolet (UV) rays from the sun. These are shorter wavelengths than the visible blue and violet and so have more energy, causing sunburn and worse. Other gases in the atmosphere absorb other wavelengths beyond our view. For example water vapor absorbs strongly at two main wavelengths in the near infrared, just beyond our vision. The absorption is so strong that no solar energy makes it to the Earth’s surface at those wavelengths; the Earth is dark there. Farther out, at thermal infrared wavelengths is the infamous ‘greenhouse effect’ of carbon dioxide and methane. Both gases absorb wavelengths emitted by the warm Earth, warming the atmosphere in the process.
Everything G lows
The range of wavelengths given off by an object like the sun is governed by its temperature. Max Planck discovered in 1900 that we could calculate the strength (amplitude) of each wavelength emitted using a relatively simple equation which produces the so-called Black-Body spectrum. The sun’s spectrum peaks in the yellow part of the visible spectrum and tails off in the UV and near-IR. Every object with a temperature above absolute zero also emits a Black-Body spectrum, the peak of which is determined by its temperature.
Objects like people and the Earth’s surface have a peak in the thermal IR, around 4 10/10,00ths of an inch (10 micrometers). That device they keep pointing at your forehead to tell your temperature is measuring your thermal IR emission. Hotter objects like the sun peak at shorter wavelengths. Thus we speak of “red-hot,” because the object has a peak emission in the red part of the spectrum, much shorter than the cooler Earth. Photographers have long known about “color temperature” and now that we’re all moving toward LED lighting, we’ve all started to learn that a color temperature of 7000 K is bluer than a temperature of 5000 K (K stands for Kelvin, another measure of temperature). Unfortunately, the subjective terms “warm” and “cool” have been attached to the opposite temperatures, but the quantitative color temperature remains.
Another glow that caused a stir when it was discovered was the faint glow of the Big Bang, which happened some 14 billion years ago. Due to the expansion of the universe since then, the glow has red-shifted to a peak representing just 3 degrees above absolute zero (3 K). That peak is way over at short radio wavelengths, but was discovered by Robert Wilson and Arno Penzias, when they were developing microwave communications back in the ‘60s.
Scientists found a long time ago that we can extend our visual use of color out to other wavelengths and use characteristic absorptions to determine compositions. For example, iron oxide (rust) is red and miners have long used that fact, along with other distinctive colors, to target their explorations. In the laboratory, scientists illuminate materials with light and use a prism to separate the colors and measure the strength or amplitude of each color, out into the near-IR. We’ve found that many minerals have distinctive absorptions at particular wavelengths, leading to the ability to fly cameras on aircraft and satellites that can differentiate and even identify those minerals on Earth and other planets remotely. We call it Remote Sensing.
Remote sensing has been used to map economic mineral deposits, ages of desert landforms, lava flows and active volcanoes, ice floes, vegetation types, crop stress, etc. etc. My colleagues tell me that studies of snow in the infrared has shown that, rather than the uniform white we see with our eyes, snow is one of the most ‘colorful’ substances – just outside our vision. When we send satellites to other planets, we use what we’ve learned on the Earth to map water-formed clay minerals on Mars, lava flows on Venus, water-ice ‘bedrock’ and methane lakes on Titan, and frozen nitrogen glaciers on Pluto.
Cooking with the Spectrum
As we continue working our way to the right on the Electromagnetic Spectrum, we exit thermal infrared and move into longer millimeter wavelengths (0.04 – 0.25 inch) also called the Extremely High Frequency band. This is an area that’s technologically difficult to work in, but the new 5G technology for cell phones is starting to use that part of the spectrum. Further to the right is the microwave part of the spectrum. Now the wavelengths range from about half an inch to 3 feet. It’s easier to generate that sort of energy and we’ve found a lot of uses for it, from radar guns that clock your speed to microwave ovens (originally called ‘radar ranges’), which operate at about 5 inch wavelength. Microwaves cook food because the water molecules within the food absorb the microwave energy and vibrate and rotate, causing heat.
Radar (“Radio Detection And Ranging”) was invented during World War II and was used to detect and track planes and missiles. Several wavelength ranges or ‘bands’ were designated with letters, but for secrecy’s sake, there’s no rhyme or reason to them. Thus K band is a very short wavelength (about half an inch), but X band is about an inch and L band is about 10 inches. Many sailboats, including mine make use of radar nowadays and that is typically X band.
Shortly after WWII, engineers developed a way to create useful images with radar sensors. This was remarkable seeing as how the resolution (how small an object you can see) of an instrument is related to its wavelength and the size of the aperture (a telescope in the case of optical and an antenna for radar). So to get a decent image at microwave wavelengths you’d need an antenna that’s several miles long! The engineers devised a system that used the fact that radar sends out its own illumination as it flies along, so they add the pulses coming back from the ground to synthesize a huge antenna. It’s called synthetic aperture radar, or SAR. Another remarkable aspect of SAR is that the resolution from an orbiting satellite is about the same as from a much lower aircraft as the satellite can add up more pulses to compensate for its greater distance.
Communicating with the Spectrum
Now we’ve moved on past the microwave into radio wavelengths. These have been exploited for a long time, as they’re pretty easy to produce and modulate. They also travel a long way, passing through most obstacles including clouds, buildings, and people. Longer wavelengths even bounce off of Earth’s ionosphere making it possible to converse with someone halfway around the world.
Since it’s been in use for such a long time, this part of the spectrum has been split up into many bands, which have been allocated for different uses, so that users don’t interfere with each other. Users include broadcast radio and TV (including satellite broadcasting), amateur radio (like the well-known 2 and 10 meter bands); terrestrial communication like cell phones, WiFi, and bluetooth; GPS satellite transmissions and the like. Some of these users have moved into shorter microwave wavelengths as the shorter the wavelength, the greater the information content. The downside of shorter microwave wavelengths is that they can be blocked by dense clouds, buildings, or people.
That concludes our short tour of the electromagnetic spectrum. We’ve spanned wavelengths from 40 trillionths of an inch (1/1000th of a nanometer) to 100’s of feet or more as we saw how we use it to see the world around us in new ways as well as a tool for communication and many other uses.