The subject of electromagnetic radiation has come up several times in previous articles, so we should probably spend a little time on it.
We are bathed in invisible waves of all kinds, pretty much all the time. If you’ve ever seen the movie “The Matrix”, in the part at the end where Neo gets up after the agents shot him, and he finally “sees” the matrix, that is sort of what the world would probably look like if we could actually see the all the waves—it would just be a huge jumble of energy, and it would probably be like looking at a blindingly bright light. Fortunately, we can only see a tiny, tiny portion of the waves that pass by us and through us constantly.
There are two main types of waves—mechanical and electromagnetic. But first, maybe we should define what we mean by “wave”.
All waves are is the propagation of energy from a source outward. When you think of a wave, what probably pops to mind is waves on the beach. Those breakers are, indeed, waves. They are mechanical waves, meaning that they represent energy moving through a physical medium (the water). They are generated, usually, by the wind blowing across the water. There is, oddly enough, friction between the air and the water, and the energy in the moving air (just like you feel the wind blowing) sort of starts to “push” the water along. The water is heavy, so once it gets moving, it has a lot of momentum and the energy in the water just travels along. Interestingly, the water itself is (usually) not moving—it’s just the energy supplied by the wind that gets transferred from molecule to molecule, much like those cool hanging ball things where one ball is dropped against another and the ball on the other end moves. All the energy of the falling ball on one end of the chain is transferred from ball to ball until it gets to the end and the last ball then swing away, because the energy has nowhere else to go. The wave in the ocean is probably barely noticeable in the open ocean, maybe generating a rolling swell, unless there is some serious wind stirring things up. If it’s really windy, sometimes the wind will blow the tops of the swells off, creating “whitecaps”, which are different from the breaking waves that occur at the shore. The energy will just go along until the water through which the energy is moving starts to run out of space, near the shore. As the floor of the ocean comes up, the energy in the water causes the water to start to “pile up” and that’s when we see the wave. It will pile up higher and higher until friction with the bottom starts to slow the lower part of the wave down while the upper part keeps going at the same speed. Eventually, the top gets far enough ahead of the bottom that gravity pulls the top part of the wave down and the wave “breaks”. If you’ve seen the pictures of tsunamis that hit land, you can see the awesome amount of energy that they carry.
I mentioned that, usually, the water itself isn’t moving, it’s just an energy wave traveling through the water. Sometimes, however, the water does actually move. The storm surge that accompanies a hurricane is an example. A strong, large hurricane can produce winds of such magnitude that they actually do end up pushing water along (together with the really large amount of energy imparted to create waves as described above). The hurricane will push these waters ashore, creating a flood that may be 10 feet, or more, in height. The storm surge that accompanies a hurricane can be even more damaging than the wind, particularly near the shore. Tsunamis, or tidal waves, are caused by undersea earthquakes. During the earthquake, a part of the ocean floor drops down or pushes up (or both). The amount of energy created by this can be incredibly large, depending on the magnitude of the earthquake. This vast amount of energy travels through the sea, and can cause huge waves near shore that can wash over the land causing immense damage. Such a tsunami is what destroyed the Fukushima nuclear power plant in Japan in 2011. The plant survived the earthquake, for the most part, but the 45-foot tsunami that hit about 50 minutes later, rolled over the seawall protecting the plant and took out the emergency power generators that provided power for the reactor cooling pumps, causing the reactors to melt down.
Tsunami
Source: NASA
Another common example of a mechanical wave is the “pebble in the water”. If you drop a pebble in the water, waves will ripple out in a circular pattern from the source of the energy (the pebble you dropped) until they hit something or spread out so far that you can no longer see the wave on the water’s surface. The waves in this example are always highest right where you drop the pebble, and get lower and lower the farther out they propagate. This is because there is “X” amount of energy in the wave. Right where you drop the pebble, the energy only pushes through a small amount of water, so the wave is relatively big. The further away from the source the wave travels, that same amount of energy is being spread through much more water, so the wave is lower. Eventually, when they get far enough away, the ripples will die out.
Source: Wikimedia Commons, Luis Nunes
Sound waves are another example of mechanical waves. When something produces a sound, it causes a wave to move through the air (or water, or whatever medium, but we’ll stay with air). That energy wave travels along until it hits your eardrum, which absorbs the energy and is pushed by it. This sets up a vibration in the bones of your middle ear, which then transfer the energy to the fluid in the cochlea in your inner ear. The wave travels through the fluid until it hits certain receptor cells which bend when the wave hits them. When these cells bend, they produce an electrical signal that travels from your ear to your brain, and your brain then “reads” the electrical signal as a sound. We’ll do an article about how the ear works sometime. It’s fascinating stuff.
The other type of waves are electromagnetic (EM) waves. EM waves are not mechanical, so they don’t require a medium to propagate through. This is why light (a form of EM energy) can travel through the vacuum of outer space and sound can’t. It’s also part of the reason why EM waves travel at close to the speed of light (300,000 kilometers per second, 186,000 miles per second), while mechanical waves are much slower. Sound waves, for example, travel at about 1200 km per hour (760 miles per hour) in air. They are slower in water, because water is much denser than air, so the waves can’t travel as fast. The difference in how the waves travel is why you see the bat hit the ball before you hear it, or you see the puff of smoke from the gun before you hear the shot, or why you see the lightning long before you hear the thunder. For every five seconds between when you see the lightning and hear the thunder, the lighting is 1 mile away. If It’s 10 seconds “flash-to-bang”, it’s two miles away. If you hear the thunder at the same time you see the lightning, it’s really, really close.
Electromagnetic force is based on the movement of electrical charges and is one of the four fundamental forces in the universe. Almost everything that we interact with is a result of EM force. Chemical bonds occur because of EM forces. Radio waves, visible light, x-rays and gamma radiation are all EM waves, and they are all fundamentally the same. They are all waves of EM energy, just at different wavelengths (frequencies). EM waves lie along what is called the EM spectrum. Very long-wavelength, low-frequency waves are at one end, and very short-wavelength, high-frequency waves are at the other. Wavelength is the distance between waves. In the pebble-in-the-pond example, the wavelength is how far apart the tops of the waves are. The frequency is how often a wave passes by. Wavelength and frequency are inversely related—long wavelengths come at low frequency and short wavelengths travel at high frequency. On the EM spectrum, the longest wavelengths are, theoretically, limited only by the size of the universe. Practically, the longest waves are a few thousand miles, in what is called the Extremely Low Frequency (ELF) radio band. ELF radio waves used to be used to communicate with submarines under water, because ELF waves can penetrate deeply into seawater and most other EM waves can’t. The US Navy used to have an ELF transmitter in Wisconsin (a long way from the ocean), so some of those ELF waves used to travel right by (and through) us. Regular AM and FM radio, TV signals, cellular phone signals, GPS information and so on are also in the relatively long-wavelength band of EM radiation. There is no telling how many radio waves are passing by, and through, us all the time. Easily thousands, if not millions.
Source: Nasa
Electromagnetic Spectrum comparing wavelengths to sizes of common objects
Next to radio waves are infrared, like in infrared lights and heaters. “Infrared” means “below red”. It’s called that because the next band of EM radiation is visible light, and red is the longest wavelength of visible light. We feel heat from infrared radiation because the water in our bodies absorbs EM radiation in the infrared range. Visible light actually contains more energy than infrared, but we don’t absorb it as well, so it doesn’t warm us as much. The “visible light” segment of the EM spectrum is just like any other part of the spectrum. It’s just called “visible light” because we have adapted sensors (our eyes) that allow us to detect EM waves in this range directly. We will also do an article on how our eyes work, sometime.
At the other end of visible light is the blue-violet light. Beyond that is ultraviolet (“beyond violet), then comes x-rays, microwaves and gamma rays. EM radiation at the UV band and above is very energetic and can cause damage to living things. UV radiation from the sun is what causes sunburn. It can also damage the DNA in cells which can lead to, for instance, skin cancer. Microwaves are energetic enough to heat up your burrito. X-rays and gamma rays are so energetic that they are referred to as “ionizing radiation” and can cause more damage to cells and DNA, particularly at high doses. Gamma rays are what kill you when a nuclear reaction occurs. You can receive enough gamma radiation to kill you without ever even knowing you were exposed. The wavelength of gamma rays, at the shortest-wavelength, highest frequency, end of the EM spectrum have wavelengths of subatomic length, is less than trillionths of a millimeter. Compare that to the many miles-long wavelength of an ELF wave. They are both electromagnetic waves, but very different in their properties, such as energy content and how well they penetrate matter.
So, now you have a little bit of understanding of waveforms and electromagnetic radiation. These are very fundamental ideas in science (particularly the EM part), because they come up again and again when we talk about all sorts of other things.
We are, indeed, constantly bathed in waves of all kinds. Fortunately, we don’t get exposed to a lot of EM radiation in the microwave, X-ray and gamma ray range, but the rest of it is pretty much everywhere, all the time. It’s probably for the best that we can’t see it all.