It lets you see or talk to a loved one in another corner of the world, and sometimes it fries you from outer space — it’s electromagnetic radiation. It’s a really nifty thing. So, let’s take a look at all the different kinds of electromagnetic radiation and why they all are, in fact, the same thing.
When charged particles in the shape of atoms (ions) or elemental particles (electrons or protons) receive enough energy to move about and interact with their peers, they start creating magnetic and electrical fields. The interplay between these two types of fields generate (you’ll never guess) electromagnetic phenomena. Which is all very good news: electromagnetism (EM) is one of the fundamental forces in nature, the set of four natural laws that took charge after the Big Bang and shaped our universe into what it is today.
One especially interesting slice of the EM pie is electromagnetic radiation. These phenomena currently hold the undisputed record for the fastest things ever. So let’s take a look at them, starting with:
Photons are probably best known for their role as the light ‘carrying’ particle, but that’s only part of their job. These elementary particles are the energy carriers for several other kinds of waves, which taken together form the electromagnetic radiative (EMR) spectrum. Like any type of wave (yes, waves on water included) they’re characterized in part by wavelength and frequency. In order of increasing frequency / decreasing wavelength, they can be:
- radio waves
- infrared radiation
- visible light
- ultraviolet radiation
- gamma rays
On first glance, they can seem to be wildly different things. Like, X-rays can be used to peer through skin, and ultraviolets give you a tan and a skin burn if you don’t use sunscreen. Totally different, right?
Well, not really. Think of the emectromagnetic radiation spectrum as a guitar string stretched over eight frets. Play the lowest note and you get radio waves, play the highest one and you get gamma rays. On a guitar, different vibrational patterns in the string will give off distinct sounds in the form of notes — our perception of them varies, but they’re all basically the same thing set on different intensity settings. Similarly, different oscillation patterns of magnetic and electrical fields will generate various kinds of EMR. We perceive them as completely different (some we can’t directly sense at all,) but they’re all basically the same phenomena on different intensities.
A source generates EM radiation when there’s energy in the system because that’s what makes particles vibrate. As a rule of thumb, hotter bodies generate waves with more power and predominantly at higher frequencies. Frequency is measured in hertz (Hz), which is defined as one cycle per second. A frequency of one Hz means one wave is generated each second, one kHz means 1,000 waves are generated per second, and one GHz corresponds to one billion per second.
Wavelength is equal to speed over frequency and is usually taken to represent the distance between two successive crests. Technically, however, it can be measured anywhere on the wave.
Lastly, electromagnetic radiation stands apart from the rest of EM phenomena in that they are ‘far-field’ effects. These waves aren’t limited to interacting with close-by objects, unlike the electrostatic effect, for example. Once generated, the waves can also hurtle through space (they ‘radiate,’ where the term ‘radiation’ comes from) without any more input from the charges that generated them. So these waves will keep going until they run out of energy — either because they hit some particles they can interact with, or because they simply fizzle out.
So now we have a basic idea of how they form, cool. Let’s go through each type of wave.
Radio waves have the lowest frequencies of all types of EMR, and its photons carry the least amount of energy. Usually, anything between 3kHz and 300 GHz is considered to be a radio wave, although some definitions class anything above 1 GHz or 3GHz as microwaves. This makes radio waves the sloths of EMR. Radio wave photons are spaced far apart — at 3khz, wavelength is 100 km (62 mi) long, 1mm (0.039 in) at 300 GHz — meaning they carry less energy than other types of ER.
Their interaction with matter is largely limited to creating a bunch of electric charges spread out over a lot of atoms — so each charge is pretty tiny. It’s useful, however, since this spreading allows a conductor tied to a circuit to transform radio waves back into some electrical signals. Couple that with their speed (all EM waves travel at the speed of light in a vacuum), and they’re really good for long-range communications.
Alternatively, if you have a conductor that isn’t tied to a circuit, say an airplane in flight, separation of those charges will generate new radio waves — this is what allows radar signals to ‘reflect’ off of stuff. The absorption or emission of radio waves always produces an electrical current, heat, or both.
Microwaves are electromagnetic radiation with frequencies between 300 MHz (wavelength 100 cm) and 300 GHz (0.1 cm). Apart from a bit more energetic photons and a shorter wavelength (which means more energy density), they’re kinda-radio-wave-ish really. In fact, microwaves are extensively used in communication as well, but with a few key differences from radio waves.
First is that you need a direct line of sight to the receiver, as microwaves don’t bend (diffract) around hills or mountains, they don’t reflect back from the ionosphere, or follow the planet’s curvature as surface waves. But they pack more of a punch than radio waves and can pierce through some of the things that radio can’t — like thick clouds or dust — due to their higher frequency.
Microwaves are used to transmit data over wireless networks, to communicate with satellite and spacecraft, in autonomous and classical vehicles for collision avoidance systems, some radio networks, keyless entry systems, and garage door remotes.
They’re also useful in ovens. The same process that allows radio wave absorption to generate heat makes a 2.45GHz (12cm) microwave very good at heating water. And since food always has at least some water, it means microwave ovens are a nifty way to heat up food.
The sweetheart of cheesy action movies, infrared, or IR. It comes just long of the visible spectrum, spanning from 300 GHz (1mm) to the lower visible limit (the color red) at 430 THz (700 nm). This is the spectrum over which most objects you’ll interact with radiate heat. Unlike radio and microwave radiation, infrared radiation interacts with dipoles (heavily polarized chemical molecules such as water), meaning it gets absorbed by a wide range of substances — and almost all organic substances — that turn its vibration into heat. However, the reverse is also true, meaning that bulk substances generally radiate some levels of IR as they release their heat.
So it’s not very good for long-range communications, since it would just get absorbed by the water in the atmosphere. But your TV remote can use IR to issue commands over short distances with great success. IR detectors are useful if you’re trying to see something that gives off heat — such as a burglar in the middle of the night. Infrared is also used in astronomy to peer through dust clouds in search of planets, in industrial applications to monitor for heat leaks or prevent overheating, in weather forecasting, and in certain medicinal applications. The military is also, obviously, a big fan of IR, using it both for observations and for guiding munitions towards a target.
And as lizard-lovers out there know, infrared radiation is a great way to beam heat where it’s needed. In fact, that’s exactly how people discovered IR. Back in 1800, an astronomer by the name of Sir William Herschel first described IR radiation by observing its effects on a thermometer.
Like all other electromagnetic radiation, IR carries energy and behaves both like a wave and like a quantum particle, the photon. A bit over half of all the solar energy that reaches Earth does so as infrared radiation — that’s why sunlight feels so warm.
This is the interval of electromagnetic radiation that your eyes are tuned to pick up. Visible light spans the spectrum from 430-770 THz (390 to 700 nm). We see different colors because certain bits of this spectrum get absorbed by objects, and the rest gets reflected. For something to appear red to you, it needs to absorb the wavelengths that don’t correspond to the color and reflect just red wavelengths for your eyes to pick up.
However, color can also arise from the way light interacts with a particular object. An object’s texture is also created by much the same mechanism. Snow, for example, appears to be white, matte and reflective at the same time — but individual snow crystals look like bits of glass. You can find out why here.
The EM spectrum over the frequency of 789 terahertz (THz) or more is called ultraviolet. Ultraviolet light is composed of really short waves, from 10 nm to 400 nm, and carry a lot of energy. In fact, starting from the UV border, photons carry enough energy to alter certain chemical bonds into new arrangements. Which is hell if you’re a DNA molecule just trying to preserve information. Even worse for living stuff, certain UV subtypes that don’t have enough energy to damage DNA directly (such as subtype A) still pose a risk because they produce reactive oxygen species inside the body, highly reactive compounds that hijack chemical bonds in DNA.
Overall, UV radiation is energetic enough that it starts being a real hazard to life. Even relatively low-energy UV can cause nasty skin burns, far worse than those caused simply by temperature (since they’re also radiation-burns, as explained above). Exposure to higher-energy UV can lead to cancer, as the waves wreak havoc on DNA strands.
This ability to damage living organisms will be a common feature from now on the list, as frequencies will only keep increasing further on. At the higher ends of the UV spectrum (around 125 nm or less, sometimes called “extreme UV”), the energy carried by these waves is so high that it can actually strip electrons from atoms’ shells in a process called photoionization.
Considering that UV radiation constitutes about 10% of the sun’s total light output, it would cause a lot of trouble for anything living on land (since water does a pretty good job of absorbing UV). Luckily for us Earthlings, we’re protected by the ozone layer and the rest of the atmosphere, which filter out most UV rays before they cause any real damage.
It’s not all bad news, however. UV radiation is key to the synthesis of vitamin D in most land vertebrates, including humans. UV rays are also used in photography and astronomy, in certain security applications (to authenticate bills or credit cards), in forensics, as a sterilizer, and of course, on tanning beds.
X-rays / Röntgen radiation
With frequencies ranging from 30 petahertz to 30 exahertz (‘peta’ means 15 zeros, ‘exa’ means 18 zeros) and wavelengths from 0.01 to 10 nanometers, X-rays are very energetic. Those with wavelengths under 0.2–0.1 nm are called ‘hard’ X-rays. Doctors use them to see the bones inside the body because they’re so tiny and powerful that our soft tissues are virtually transparent to them. Same goes with luggage at the airport — hard X-rays can see right through them. Their wavelength is comparable to the size of individual atoms, which is why geologists use them to determine crystal structures.
X-rays (and the more energetic gamma rays) are made up of photons that all carry minimum-ionization energy (they can all photoionize), and are thus called ionizing radiation. They can inflict massive damage on organisms and biomolecules, often affecting tissues very deeply below the skin as they easily penetrate through most matter.
They are named after Wilhelm Röntgen, the German scientist who discovered them on November 8, 1895. Röntgen himself called them X-radiation because it was quite mysterious at the time — nobody really understood what this radiation was or what it did.
These are the EMRs with the single highest-energy photons we know of. They have frequencies in excess of 30 exahertz, and wavelengths of under 10 picometers (1 picometer is a thousandth of a nanometer or a thousandth of a billionth of a meter), which is less than the diameter of an atom. They’re mostly resulted from radioactive decay here on Earth (like nukes or Chernobyl), but can also come in ridiculously powerful gamma-ray bursts, likely the product of dying stars going supernova or the larger hypernova before collapsing into neutron stars or black holes. They are the single most deadly type of EM radiation for living organisms. Luckily, they’re largely absorbed by Earth’s atmosphere.
Artificial gamma rays are sometimes used to alter the appearance of gemstones, such as turning white topaz into blue topaz. The US is also experimenting with using them to create a sort-of X-ray machine on steroids that can scan up to 30 containers per hour. To get an idea of how ridiculously penetrative gamma rays are, know that mining operations use gamma ray generators to look through huge piles of ore and select the richest for processing. Other uses include irradiation (used to sterilize medical equipment or foodstuffs), to kill cancer tumors, and in nuclear medicine.
In short, these are the categories we use to describe electromagnetic radiation. They have things they like to pass through, and things that they reflect from. They’re the light you can’t see and can be pleasant, very dangerous, and sometimes, insanely deadly.