The Electromagnetic Spectrum

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Podcast Transcript

All around you, right this second, you are surrounded by electromagnetic radiation. 

You might better know this by names such as light, radio waves, microwaves, x-rays, or ultraviolet rays. 

Fundamentally, they are all variations of the same phenomenon and are all part of the electromagnetic spectrum. 

Learn more about the electromagnetic spectrum and how different wavelengths can behave very differently on this episode of Everything Everywhere Daily.

As I mentioned in the introduction, electromagnetic radiation has many different names. 

Radio waves, microwaves, infrared and ultraviolet radiation, x-rays, and gamma rays are all different manifestations of light. Visible light that we can see makes up a very small part of the total spectrum. For the purposes of this episode, when I refer to “light,” I’ll be referring to any electromagnetic radiation. 

Electromagnetic radiation has properties that are similar to a particle, and they also have properties that are similar to waves. 

For the purposes of this episode, I’ll mostly be speaking in terms of waves, but I will occasionally reference a photon. A photon is just a single unit, or quanta, of energy that is emitted. 

The length of the wave, or wavelength, defines the electromagnetic spectrum. If you can imagine a sine wave that goes up and down, the peaks in the wave grow farther apart if you stretch it out. They have a longer wavelength. 

The wave has a shorter wavelength if you compress the wave. 

The wavelength of light is inexorably tied up with the wave’s frequency. The frequency is just the number of times the wave goes up and down per second, and it is measured in hertz. 

A 2 megahertz radio signal will have a wave that goes up and down 2 million times per second. A 30-hertz wave will only go up and down 30 times per second. 

The relationship between frequency and wavelength is one of the simplest equations in physics. Frequency times wavelength will always equal the speed of light. 

Also of note is that the longer the wavelength, the less energy a photon will contain. Very short wavelengths, like gamma rays, contain an enormous amount of energy and can be very dangerous. More on that in a bit. 

So, the electromagnetic spectrum is just all of the different wavelengths of light. 

I should note that there is technically no theoretical maximum for the wavelength of a light wave. There are practical limitations, however. A wavelength the size of the solar system would have photons with so little energy it would be near impossible to detect them from cosmic background radiation.

There is a minimum wavelength. It would be a light wave with a frequency that is known as the Planck Frequency. It would be a 2×1043 hz wave. At that point, each photon would become a black hole, which is far beyond what we need to worry about in this episode.

Let’s start our trip down the spectrum with what is known as Extremely Low Frequency radio waves, or ELF.  ELF waves are between 3 and 30 hertz and have a wavelength of 100,000 to 10,000 kilometers. 

This is pretty much of limit of what humans can reasonably work with.

The only real use for ELF waves is communicating with submarines while they are deep in the water.  

The United States used to run an ELF transmitter in Northern Wisconsin for this purpose. The antennas were just powerlines that ran 14 miles or 23 kilometers in each direction of the transmitting station. The antennas were far smaller than the wavelength they worked with, but you can get by using an antenna with a fraction wavelength. 

Just above these are Super Low Frequency waves. These are between 30 and 300 hertz and have wavelengths between ??10,000 to 1,000 kilometers and are also used for submarine communication. 

I should note that there are no hard and fast boundaries between the parts of the spectrum I’ll be referring to. They are all arbitrary and can be divided or combined as is helpful.

Ultra Low Frequency waves are between 300 and 3000 Hz and have wavelengths of 1,000 to 100 kilometers. Again, this part of the spectrum isn’t that useful, but it can penetrate the ground. There have been radio systems for communicating with mines that have used this frequency.

Next are Very Low Frequency waves, which are between 3,000 and 30,000 Hz, or 3 to 30 kHz. The wavelengths are 100 to 10 kilometers. 

This is now starting to get into frequencies that have practical purposes. Long wavelengths of radio waves can travel great distances but have very low amounts of bandwidth available. The very low frequency spectrum is used for things like radio navigation, seldom for voice. 

From 30 to 300 kHz are Low Frequency waves with wavelengths from 10 to 1 kilometer.  There are certain radio stations known as “longwave” radio that uses this part of the spectrum. It is also used for aircraft navigation and time signals. 

I have a clock above my computer as I’m typing this that is synchronized to the atomic clock in Fort Collins, Colorado, that picks up a time signal at 60 kHz. For those of you who are geeky, the information sent to clocks is sent at a rate of 1 bit per second. The complete time code is 60 bits so it takes a minute to send the complete set of data.

From 300 kHz to 3 MHz is the Medium Frequency part of the spectrum, which has wavelengths from 1 kilometer to 100 meters. 

This is the part of the band where you will find AM radio stations in the United States. In the US. AM radio is found between 540 kHz and 1,700 kHz. 

In this range, you will also find more navigation beacons as well as air traffic control radios. 

Above this, we find High Frequency radio. The names are a bit odd because high frequency is actually quite low, considering what is possible. The HF bands go from 3 to 30 mhz and have wavelengths from 100 to 10 meters. 

High frequency radio waves have a special property in that they can be reflected back to Earth by the ionosphere. This allows for radio waves that can travel very long distances, especially at night.  This is the realm of shortwave radio. 

Shortwave radio was used heavily during the Cold War to get radio signals into other countries. There are still shortwave radio stations that exist and it can be fun to try and pick them up if you have the right receiver. 

Amateur radio, or ham radio, operators also use high frequency radio to talk to people far away.

Citizen Band, or CB radio, can be found in the 11 MHZ frequency range. 

Between 30 to 300 MHz, we have Very High Frequency or VHF radio, which has wavelengths from 10 to 1 meter. 

VHF radio only works on line of sight, so it is only good for short-distance communications. Amateur radio operators use VHF radios, and VHF is also the primary part of the spectrum for terrestrial television. 

In the United States, FM radio stations also operate between 87.5 to 108 MHz.

Next is Ultra High Frequency or UHF radio. This has a range of 300 MHZ and 3 gHz with wavelengths of 1 meter to 10 centimeters. 

UHF signals are used for a wide variety of things, including television, GPS, mobile phones, wifi, radar, and a host of other applications. 

The UHF part of the spectrum is subdivided into many smaller sections for specific applications. Signals in this range can’t travel very far and are also line of sight. However, they can carry much more information than lower frequency signals can. 

The 2.4 gigahertz region became so popular because it wasn’t actually assigned to anything. Because it was unlicensed, anyone could use it, and a whole host of applications found themselves there. 

Once you get to this part of the spectrum, because the waves don’t travel as far, you don’t have to worry as much about interferences as you would with TV or radio stations. 

Above UHF, we have Super High Frequency or SHF signals. They operate at 3 to 30 GHz and have wavelengths of 10 centimeters to 1 centimeter. 

This is the band where you will find microwaves which are great for narrowly focused point-to-point communications. You might have seen microwave antennas on a radio tower that look like drums. 

You might have some experience with this if you have ever had to switch between 2.4 GHz wifi and 5 GHz wifi. The 5 GHz signal is faster and has more bandwidth, but it doesn’t travel as well through walls. If you are ever in a hotel or even parts of your own home, a 2.4 GHz signal might be better if you can’t pick up a faster 5 GHz signal. 

Microwave ovens operate around 25 to 38 millimeters. 

Above this, we have Extremely high frequency or EHF signals. They have a frequency of 30 to 300 GHz and wavelengths of 10 millimeters to 1 millimeter. 

We are now back into a part of the spectrum which isn’t very useful. Waves in this region can be absorbed by the atmosphere, which doesn’t make them useful for communications. 

At the bottom end of this range, there are some 5G signals around 24-54 GHz, but the signals can only travel very short distances. 

In certain parts of this range, signals can’t be sent more than a meter before they are totally absorbed. 

There are some applications here for scientific instruments and those millimeter wave security scanners you walk through at the airport where you have to put your hands in the air. 

Above this, we enter the realm of infrared radiation. The definitions of the spectrum boundaries beyond this become a bit messier as there isn’t a need to regulate this spectrum. I’ve come across several different ways to categorize the infrared spectrum, and the one I’ll use is pretty broad.

Far Infrared radiation has a frequency of 300 GHz to 3 THz with wavelengths of 1 millimeter to 0.1 millimeters. The biggest use of this part of the spectrum is in astronomy and in infrared sensors, those that turn on and off lights when you enter a room. 

Mid Infrared Radiation has a frequency from 3 to 30 THz and a wavelength of 100 to 10 micrometers

Near Infrared Radiation includes everything from about 30 THz to 400 THz. 

Infrared radiation is really important because is basically heat. It is given off by planetary bodies and by your body. The James Webb Telescope is tuned to view infrared light. Infrared light is the preferred light for use in many fiber optic cables because it travels farther in glass. 

Night vision goggles and cameras capture infrared light.

As I said, the boundaries between these types of radiation can be defined differently, but the upper boundary is very clear. 

Just above the infrared part of the spectrum is Visible Light. I will probably do a future episode on this because there is a lot about the science of colors, rainbows, and prisms that is really interesting.

The visible part of the spectrum extends from 400 THz for red to 790 THz for violet. The wavelengths go from 625 to 400 nanometers. 

Just beyond the color violet lies the next part of the spectrum, the Ultraviolet or UV.

Ultraviolet has a wavelength of 400 nanometers to 10 nanometers. About 10% of the energy given off by the sun is in UV radiation. 

Longer wave UV radiation can actually cause chemical reactions and it is used by our bodies in the creation of vitamin D. 

Shorter wave UV radiation, also known as extreme ultraviolet, can start to do damage to cells and DNA, and UV radiation is often used to sterilize medical instruments and water. Because we can’t see UV radiation, if you have ever used a UV sterilizer, they will often add a blue light to it just so you can see that it is working.

Extreme ultraviolet is the beginning of what is known as ionizing radiation. As I mentioned before, the higher the frequency and the shorter the wavelength of light, the more energy an individual photon contains.

At this point and beyond, the photos have enough energy to do damage to biological organisms. 

After UV we find X-Rays

X-rays cover the range from 10 nanometers to 10 picometers and have frequencies of 30 petahertz to 30 exahertz. 

X-rays were discovered by Wilhelm Röntgen in 1895 when images appeared on photographic film when stored in a desk with radioactive material. 

X-rays have so much power they can go through things, which is why they are used for imaging inside of solid objects.

Because it is considered to be ionizing radiation, exposure to x-rays has to be limited. If you have ever gotten an x-ray, you’ll notice that the technician is almost always in another room when they activate it. 

Our last stop on the electromagnetic spectrum are Gamma Rays

Gamma Rays are dangerous, and they are one of the three types of radioactive decay from radioactive elements. 

Pretty much anything with a frequency over 30 exahertz or a wavelength less than 10 picometers is considered a gamma ray. 

Gamma rays have wavelengths so small, you can’t build a mirror to reflect them, because they would pass right through. They are also very difficult to detect for the same reason. 

They have uses in for certain sterilization procedures and aggressive cancer treatments. There is a branch of astronomy that tries to observe gamma rays that are emitted by high energy objects. 

They can also be produced in limited amounts by thunderstorms and they also hit the Earth in the form of cosmic rays.

So, as you can see, even though all the different parts of the spectrum are waves of electromagnetic energy, how they interact with the world and how useful they are is totally dependent on their frequency and wavelength. 

The part of the spectrum which is good for talking to submarines isn’t good for wifi, and visible light isn’t very good for getting images of the inside of your body. 

The really useful parts of the spectrum from low frequency through microwaves are all regulated through what is known as a spectrum plan. Different groups are given different parts of the spectrum for different purposes.

However, these allocations can change over time. 

For example, taxi drivers used to communicate regularly via radio. However, you don’t see as many taxi drivers using radios anymore. They just use cell phones which are much cheaper and easier. 

The spectrum which was allocated to taxis was then able to be reallocated to something else. 

There are both national and international rules for how spectrum can be used and allocated, but that too is for another episode. 

All of us, in one way or another, have used and interacted with different parts of the electromagnetic spectrum every day of our lives.

Understanding the different parts of the spectrum and what they can do is a worthwhile thing to know for everyone.