Video Transcript
In this video, we’re talking about
applications of electromagnetic waves. In our everyday life, there are
already dozens of places where electromagnetic waves play apart. Any time we turn on a television,
send or receive a text, or heat up some food in a microwave, we’re using
electromagnetic waves. In this lesson, we’ll talk about
some of the leading applications and the kinds of waves they correspond to.
To get started, one of the first
points we can make about electromagnetic waves, sometimes called EM waves for short,
is that they carry energy. This means that as an
electromagnetic wave travels along, it transmits energy from one spot to
another. The amount of energy a particular
wave transmits has to do with its wavelength, the distance from one peak of the wave
to an adjacent peak, or equivalently from one trough to the next trough over. The wavelength, this straight-line
distance between peaks or troughs, can be longer as it is for this wave, or shorter
as it is for this one. The shorter the wavelength, the
more energy a wave has. So, this wave in pink, because it
has a shorter wavelength, has more energy than the wave in blue.
Now, when it comes to the
wavelength of electromagnetic waves, there’s no particular upper or lower limit. A wave’s length could be as short
or as long as we could imagine. If we were to organize all
electromagnetic waves by their wavelength, then we would have what’s called a
spectrum of waves. In particular, we would have what’s
called the EM, or electromagnetic, spectrum. This spectrum can be organized
visually by putting waves with a shorter wavelength toward one end, in this case
we’re putting them towards the left end of our spectrum, and waves with a longer
wavelength are towards the other end, in this case towards the right.
To further organize the
electromagnetic spectrum, it’s been useful to divide it up into seven regions. These regions include visible
light, the light our eyes are sensitive to, then to the right of that at longer
wavelength is infrared radiation, abbreviated IR. Then, to the right of that come
microwaves. And then finally, radio waves,
those waves with the longest wavelength of any on the spectrum.
If we go back to visible light and
move the opposite direction on the spectrum towards shorter wavelengths, we
encounter the UV or ultraviolet region. Then, we get into the X-ray portion
of the spectrum. And finally, there are gamma rays,
those waves with the shortest wavelengths of all. Notice that even though the
spectrum is divided up at the ends, at the far left and at the far right, there’s no
end point. This is due to what we noticed
earlier, that there was no lower or upper limit to the wavelength an EM wave can
have.
Now that we have all these regions
arranged by wavelength, let’s recall that energy goes down as wavelength goes
up. That is, as we get towards the
radio wave end of our spectrum, where the wavelength is very long, the energy in
those waves is very low. And then, the opposite happens as
we move towards the other end, towards gamma rays. Headed in his direction, our
wavelengths are getting shorter and shorter, and the wave energy is going higher and
higher. The applications for which these
various parts of the spectrum are most useful have to do with how much energy those
waves carry. Let’s start by considering the
least energetic waves, radio waves, and consider what they may be useful for.
Let’s say that this is the
Earth. This is Earth atmosphere, and we
want to communicate from one spot on Earth surface to another that’s out of line of
sight from the first location. In other words, we want to
communicate from here to here. But if we try to send a
straight-line signal, it will run into the earth. This is where radio waves can come
in very useful. When these waves travel through the
atmosphere, they’re capable of bouncing off of atmospheric layers and back to
Earth. This enables sending radio signals
by reflection, like we see here.
Or using other radio waves, it’s
possible to create a wave that bends inside Earth atmosphere. This is called refraction. Whether through reflection or
refraction, it’s possible to use radio waves to accomplish long-distance, wireless
communication. This application of radio waves has
become so common that the devices we typically use to receive these waves are called
radios.
Now, if we then shift over to the
microwave region of the spectrum, we once more encounter a category of wave with an
application that’s so common it’s named after the wave. We’re talking here about
microwaves, as in microwave ovens. When we put food in such an oven,
it’s indeed exposed to microwave radiation. The wavelength of this radiation is
a wonderful match for exciting water molecules, heating them up. And since water is in virtually all
the food we eat, heating up the water in food effectively heats the food. For this to work, the incoming
microwaves need to be absorbed by the food. That’s how energy is transmitted
into it. Though there are definitely other
applications for microwave radiation, this one, heating food, is one of the most
common ones.
When we go from the microwave to
the infrared region of the spectrum, the light we’re considering is still invisible
to our eye. We’re not able to see it, but
nevertheless it has some very real physical effects. For example, imagine standing near
a campfire in order to warm up. Now, we know that light from the
campfire, visible light, is available to our eyes, but there is another type of
radiation responsible for the warm feeling of the campfire on our hands. That’s infrared radiation, IR. The warming effects of the campfire
and many other heat sources are due to these invisible electromagnetic waves.
This, for example, is how radiators
in many homes work. If we’d look at a radiator while
it’s in action, we can see it’s not emitting any visible light, but it is giving off
heat. And the way it does that is through
infrared radiation. By the way, along with warming
objects, there is another common application for infrared radiation. Whenever we use a remote control to
do something like turn on or off a television, we’re using infrared radiation. The signal that sent from the
remote is infrared, invisible to our eyes, but very real nonetheless.
Now, when we move on to the visible
portion of the spectrum, arguably, this is the portion where there are the most
applications for this kind of light. This is because visible light by
its nature is so useful to us. So, of the many applications of
visible light we could pick, here is one. It’s possible to take visible light
in one location, say over here, and reproduce it in another, say over here, using a
device called an optical fiber.
If we were to look closely at one
end of this fiber, we would see that the core of it, the center, is made of glass
and the glass is covered in a coating. This glass core is what serves as a
channel, a conduit, for the visible light from one place to another. The fiber is designed so that no
matter how it bends or twists between these two locations, the light is faithfully
reproduced at the other end. This transmission of visible light
can happen over distances of thousands of kilometers. And because this communication
happens with visible light, the images that go into and come out of the fiber or
collection of fibers can be seen by our eye. Fiber-optic signaling is a
long-distance communication application for visible light.
Moving on to ultraviolet radiation
or UV for short, we now cross back over into invisible light, light our eyes can’t
see. Now, ultraviolet radiation is often
thought of as hazardous. And here is the reason why. We can see that as we move from
right to left in the ultraviolet region of the spectrum, the energy of these rays
gets higher and higher. Now, if we think about these
increasingly energetic waves interacting with an atom, here is an atom with its
nucleus and then the surrounding electrons, as the incoming radiation shown here
gets higher and higher in energy, it becomes more and more capable of ionizing this
atom.
In other words, when this ray is
absorbed by an electron in the atom, that electron takes on the energy of the wave
and now has so much energy that it leaves the atom entirely. And after it’s gone, what’s left
over is a charged atom, an ion. The point at which incoming
radiation becomes energetic enough to strip an electron away from an atom is
considered significant. If the wave can strip an electron
away, it’s called ionizing radiation because it’s capable of creating an ion. But if it’s not energetic enough,
it’s called nonionizing. And the boundary between ionizing
and nonionizing radiation falls in the ultraviolet portion of the spectrum.
So, some ultraviolet radiation with
wavelengths in this region is incapable of stripping electrons away from atoms and
therefore is nonionizing. But some UV rays in this region
here are high-energy enough to do this. An ionizing radiation is considered
hazardous because of the damage it can do to human cells. This is why, for example, the use
of sunscreen is encouraged whenever people are outside in direct sunlight for long
periods of time. It’s to shield our skin from the
ionizing radiation given off by the sun. As we’ll see later, ionizing
radiation can actually be useful for health applications. But it’s not directly related to
the particular ultraviolet application we’ll name.
At one time or another, many of us
have had the experience of being in a room where the only light supplied was coming
from what was called a black light. Typically, black lights don’t
actually look black, but have a violet or a purple hue to them. That’s because they occupy roughly
this portion of the EM spectrum. Some of the light they give off is
low-energy, visible light. That’s in the violet end of that
spectrum. But most of it is low-energy,
nonionizing ultraviolet radiation.
Now, in the presence of a black
light, any clothing that’s white in color takes on a very interesting
appearance. It looks like it’s glowing. This effect is due to what’s called
fluorescence. Fluorescence is a two-part process
that has to do with ultraviolet radiation shining on something, that object
absorbing the radiation and then being energized itself, starting to emit light,
typically, of a lower wavelength than the UV light that reached it. Our eyes aren’t able to see the
ultraviolet light that energizes the objects that end up fluorescent, but we often
can see the vibrant colors that are given off by these energized objects.
Fluorescence is often used to
create lighting in office buildings. Overhead lights that look like long
tubes have ultraviolet radiation inside them that reaches the inner surface of these
tubes. These surfaces are coated in a
phosphor which gives off visible light when energized by ultraviolet light. Ultraviolet radiation is absorbed
by the material in the tube while visible light is given off to the outside.
As we move on to the X-ray portion
of the EM spectrum, we see that the wavelength of our waves is increasingly getting
shorter while the energy is going up. X-rays are energetic enough that
they’re capable of penetrating through different kinds of matter. For example, if we send a lot of
X-rays into our hand, then only the bones in the hand are dense enough to absorb and
block the X-rays. The X-rays that don’t hit bone are
able to pass right through. This, of course, makes X-rays
useful for medical imaging.
But they’re useful in other ways as
well, say, at the security checkpoint at an airport. Here, X-rays are commonly used to
see into the interiors, the insides, of boxes and suitcases. Once again, the very high energy of
these waves means they’re only capable of being blocked by something fairly
dense. They can travel right through the
outside of luggage. Whether used for health or safety
or something else, imaging the interior of different objects is an important
application of X-rays.
Last but not least, let’s consider
an application of gamma rays, the highest-energy radiation on the EM spectrum. All gamma rays are ionizing,
meaning they’re all capable of stripping an electron away from an atom and creating
a charged particle. As such, gamma rays are capable of
doing a lot of damage to human cells. At first, this might not sound like
a good thing, but we can keep in mind that not all cells are good cells. When cells in our body mutate to
become something they shouldn’t and then start to rapidly divide, this can be the
beginning of the development of a cancerous tumor.
Before the tumor continues to grow
for much longer, we’d like to destroy it by destroying the cells it’s made up
of. This is where an application of
gamma radiation comes in. Because these waves are so high
energy, when they interact with a cell, a typical response is for the cell to
die. In this case, with the cells
dividing in an out-of-control way, that’s a good thing. Through careful application, gamma
ray therapy is capable of destroying a cancerous tumor. In this way, the ionizing property
of gamma rays have been put to useful application.
As we step back and consider these
different applications for different kinds of electromagnetic waves, it’s worth
reminding ourselves that this is just a small sampling of the long list of ways that
EM waves are used. That said, let’s summarize what
we’ve learned about the applications of electromagnetic waves. Starting off, we saw that
electromagnetic waves, called EM waves for short, carry energy from one place to
another. We also learned that wave energy is
inversely proportional to wavelength. That is, as the wavelength of a
wave increases, its energy decreases and vice versa. We saw that the EM spectrum is
commonly divided into seven regions and that each one of these seven regions has an
application it’s known for.
