Video Transcript
In this video, we’re talking about
electromagnetic waves for communication. As we’ll see, there are many ways
that these types of waves are used to send a signal from one place to another. Whenever we send a text, power on a
television, or use a GPS positioning device, we’re communicating using
electromagnetic waves.
As we get started on this topic,
let’s recall a bit about how these waves work. Sometimes called EM waves for
short, electromagnetic waves are periodic oscillations that carry energy from one
location to another. That fact is one of the key reasons
why electromagnetic waves are so useful for communication.
Another reality about
electromagnetic waves is that they’re capable of traveling through empty space. In other words, these waves don’t
require a medium to get from one spot to another. Even if there’s no matter, no
material, between a starting and ending location, electromagnetic waves are still
able to travel between them.
Another thing we can note about
these waves is that they’re classified by their wavelength. That’s the straight line distance
from a crest of the wave to an adjacent crest, or equivalently the straight line
distance from a wave trough to the next trough over. The shorter the wavelength of a
wave, the more energy it carries. And interestingly, there seems to
be no upper or lower limit for the wavelength an electromagnetic wave can have.
The name for all those possible
wavelengths put in order is the electromagnetic spectrum. To help organize all these waves
further, the spectrum has been divided up into seven regions. With wavelength increasing from
left to right, the leftmost division, the one with the shortest wavelength, is gamma
rays. Next come X-rays, then ultraviolet
radiation, then visible light, the light our eyes are sensitive to. Then as wavelength continues to
increase, we get to infrared radiation, IR for short, then microwaves and, finally,
radio waves, the waves with the longest wavelengths.
Notice that there’s no upper limit
for the wavelength of a radio wave. And at the other end of the
spectrum, there’s no lower limit for that of a gamma ray. Now when it comes to
electromagnetic waves for communication, there are two factors to keep in mind. One is the wave itself, its
wavelength and therefore its energy. But the other factor is the
environment that the wave needs to travel through.
Let’s say that this is a sketch of
Earth’s outline and that this dashed line shows us Earth’s atmosphere. Now at any given time, all kinds of
waves with all kinds of wavelengths are coming from space and entering Earth’s
atmosphere. But not all of these waves make it
to Earth’s surface. A number are blocked and filtered
out. This happens largely due to a
particular layer in Earth’s atmosphere called the ionosphere.
This upper layer of the atmosphere
consists of ions, charged particles that are capable of absorbing and reflecting
certain kinds of radiation incident on them. The effect of all this is to block
that radiation from ever reaching Earth. The types of waves that the
ionosphere blocks are gamma rays, X-rays, and a number of ultraviolet rays. All in all, this is a very good
thing because this radiation is powerful enough that it would be dangerous to us if
it reached Earth. So the ionosphere is a protective
barrier. And we can see that it will affect
our choice of electromagnetic waves for communication.
For example, if we wanted to
communicate from Earth to something outside of Earth’s atmosphere, such as a
satellite, we wouldn’t want to use gamma rays, X-rays, or ultraviolet radiation to
do it. Those waves will be unlikely to
make it through the ionosphere. But as we move to the other end of
the electromagnetic spectrum, towards microwaves and radio waves, we find that these
wavelengths interact differently with the atmosphere. For example, if we have a radio
tower on the surface of the Earth and this tower sends out a microwave signal, those
wavelengths are effective and making it through all the layers of the atmosphere to
space.
For this reason, we use microwaves
to communicate with satellites, space probes, or anything else outside of Earth’s
atmosphere. Waves like this that make it all
the way from the earth to outside of Earth’s atmosphere are called space waves. But what if, instead of sending a
signal to space, we wanted to communicate with some other spot on Earth’s
surface? And let’s imagine further that that
location is out of our line of sight. In other words, if we try to
communicate directly to it, then the wave we use to do that would run into the Earth
and be blocked. In this case, we could use a type
of radio wave which is known to reflect from within the ionosphere back down to
Earth. This kind of wave is called a
skywave. And this means it comes from a
certain part of the radio wave spectrum, called shortwave radio.
Along with space waves and
skywaves, there’s another type of wave we can consider. This one has a wavelength even
longer than either of the previous two. It’s called a groundwave. But this doesn’t mean it travels
along the ground. Rather, it means that this wave
either bends through the lower layers of the atmosphere to curl around the Earth as
it were or it reflects off of the bottom of the ionosphere, never entering that
layer. Waves of this type are known as
long-wave radio.
When it comes to long-distance
communication through the atmosphere, microwaves and radio waves are the most
commonly used kinds. But we don’t want to leave out
visible light and infrared radiation because they have communication uses too. For example, visible light can be
used to communicate over long distances through a fiber known as an optical
fiber.
If we took an up-close view at the
end of an optical fiber, we would see glass in the middle surrounded by a
coating. This glass serves as a guide so
that any visible light that comes in one end of the fiber is safely transmitted out
the other end. And these fibers can be used to
communicate visible light over distances of meters up to thousands of
kilometers. In fact, we use fiber optic cables
to communicate between continents on Earth underneath the oceans. So visible light, if we constrain
its path properly, is useful for communication over long distances.
When it comes to infrared
radiation, this is in a bit of a different category because our eyes, while
sensitive to visible light, can’t see infrared. So even though optical fibers could
be designed to transmit infrared radiation, it wouldn’t be nearly so useful because
our eyes can’t see it. Nonetheless, we do use infrared to
communicate over short distances. Many of the remote controls we use
use infrared radiation to send signals to command a television, turn on or off,
change the channel, et cetera. In this application, the
invisibility of infrared radiation to our eye is actually a benefit.
So taking a step back, we can see
that, thanks largely to the constraints our atmosphere puts on what kinds of waves
we can use for communication, the electromagnetic waves we use predominantly are
visible light, infrared radiation, microwaves, and radio waves. And in particular, it’s microwaves
and radio waves which are useful for wireless communication over long distances.
Having seen all this, we may wonder
just how it is that these signals, the microwaves and radio waves we send over long
distances, are produced. To show that, let’s clear a bit of
space on screen. The radio waves and microwaves we
use for communication are produced by electric circuits. Here we have such a circuit with an
antenna as well as an AC, or alternating current, power supply.
Now it’s actually very important
that we have an alternating current in the circuit because that’s what will let us
produce a signal. In step with that alternating or
constantly switching back and forth current, the electrons in our antenna oscillate
up and down up and down very rapidly. And it’s this oscillation that
creates the signal the antenna transmits.
Note that, in sketching in this
signal, we’ve represented the wavefronts using segments of concentric circles. But we just as well could’ve drawn
in representative waves that travel out from our antenna. So then based on the current
running around this circuit, our antenna produces a signal. So we have our source, and the
source is called a transmitter.
For communication to happen though,
we need something to receive this signal. And for that, we’ll need a second
electric circuit, this one called a receiver, where the electrons in this antenna
pick up the particular wavelength of the signal that’s reaching the antenna and
start to oscillate in a similar pattern. This is how microwave or radio wave
transmission works.
Now one thing worth mentioning here
is that just as every electromagnetic wave has a wavelength, it also has something
called a frequency. Wave frequency is defined as the
number of cycles a wave goes through in one second of time. Wave frequency is related to
wavelength. The shorter the wavelength of an EM
wave, the higher its frequency.
We bring all this up because
there’s a certain technique for transmitting radio waves that involves changing the
wave frequency. In this approach, rather than
creating a wave with one constant wavelength and one constant frequency, a wave is
produced where the wavelength, the distance from one crest to another, varies. And along with this, the frequency
of the wave changes too. Whenever we see the term FM radio,
that’s what this term refers to. FM stands for frequency modulation,
changing the frequency of a wave within the wave in order to package more
information in the signal. Whenever we see FM on a radio dial,
this is what that’s referring to. The frequency of radio waves is
being modulated, changed, so the waves carry more information. Knowing all this now, let’s get a
bit of practice with these ideas through an example exercise.
Which of the following types of
electromagnetic waves is not used for sending communication signals? Infrared waves, microwaves,
visible-light waves, ultraviolet waves, radio waves.
Okay, so here we want to
identify which of these five options is not used for sending communication
signals. Now theoretically, any type of
wave could be useful in some application for communicating. But in this question, we’re
thinking on a practical level, how different kinds of EM waves are actually used
currently. We can start at the top of the
list and go down, thinking in each case how that particular kind of wave is used
or not used in communication.
Starting off with infrared
waves, this kind of wave is not typically used to communicate over long
distances. But it has been found to be
useful for communicating wirelessly over short distances. Think, for example, of any
remote controls we might use. When they’re powered on and
working properly, when we press a button on the remote, the remote sends an
invisible-to-our-eye signal, an infrared signal, to some receiver. We might be sending a signal to
power on a monitor, turn off a television, or open our garage door. But regardless of the
particular command, we’re sending it using an infrared wave. So there is a communication use
for infrared waves, which means we won’t use it as our answer.
Next, let’s consider
microwaves. We might first associate these
kinds of waves with a microwave oven, something to heat food. Indeed, that’s one of the most
common applications of these waves. But it’s not the only thing
they’re useful for. Microwaves, when sent from
Earth, are very effective and making it through Earth’s atmosphere into
space. These waves are unlikely to be
blocked or reflected back. For this reason, it’s common to
use microwaves to send signals to and receive signals from anything outside of
Earth’s atmosphere. Since microwaves do have a
communication use, we’ll cross these off our list as well.
Next, let’s consider
visible-light waves. Unlike microwaves, visible
light is not useful for long-distance communication through the open
atmosphere. But with the right constraint,
visible light can be useful for sending communication signals. One way this happens is through
what is called an optical fiber. An optical fiber is a long wire
that has a glass core, a glass interior to it. This interior is covered in a
coating. And the way it works is, light
is accepted through one end of the fiber and then, regardless of the twists and
turns the fiber might go through, that light is safely communicated out the
other end. This communication distance
from one end of the fiber to the other can be on the order of centimeters or
thousands of kilometers. So visible light can be used
for long-distance communication so long as it’s traveling through a fiber
optic. This means that visible-light
signals are useful for communication. So we won’t choose that as our
answer either.
Next up is ultraviolet
waves. These are waves that in part
are absorbed by Earth’s atmosphere. And not only do they have
trouble traveling through the atmosphere, but these waves are also invisible to
the human eye. So even if we put an
ultraviolet signal into one end of an optical fiber, we couldn’t see what would
come out the other end. Along with all this,
ultraviolet waves are high-energy enough so that some of them are dangerous to
humans. They can negatively affect the
DNA in our cells and therefore are often avoided. It looks like ultraviolet waves
will be the wave type that are not used for sending communication signals.
To make sure this is the case,
let’s consider our last option, radio waves. In this case, we can easily
think of applications of radio waves for communication. The types of radio stations
we’re able to tune into using a radio in our car or in our computer are proof
that radio waves are used for communication. So then we can finalize our
choice of ultraviolet waves as the type of electromagnetic waves that are not
used for sending communication signals.
Let’s consider now what we’ve
learned about electromagnetic waves for communication. In this lesson, we first saw that
electromagnetic waves, called EM waves for short, transport energy. And the amount of energy they
transport depends on their wavelength. Of the various types of EM waves,
we saw that radio waves and microwaves are used for wireless communication through
the atmosphere. We then learned that visible-light
communication happens through wires called optical fibers. And further, we saw that infrared
waves are useful for sending wireless signals over relatively short distances on the
order of a few meters. Lastly, we learned that radio waves
and microwaves are created by transmitter and received by receiver circuits.
