Video Transcript
In this lesson, we’ll learn how to
analyze the electromagnetic spectrum by identifying and describing types of
electromagnetic radiation and their sources. As we’ll see, once we understand
this spectrum, we’ll understand lots of the physical phenomena going on around
us.
We can start off by considering
this question: Where does light come from? We know that the Sun creates light,
so does a light bulb, and so does a television or a computer monitor. Even though there are lots of
different sources of light, there is a basic physical phenomenon that ties all these
sources together. If we go down to the level of
individual atoms, that mechanism is the acceleration of electric charge, in
particular electrons, as they change energy levels within an atom.
When such a transition happens,
when an electron moves from one energy level to another, this is often accompanied
by the emission of a little packet of light called a photon. When a photon is emitted, its
properties, such as its wavelength and its energy level, depend on just how it was
produced, how big of a transition the electron went through, or — along a similar
line — how much of an acceleration the electron experienced.
Now, there’s a very specific reason
we’re talking about photons and light. If we were to take a closer look at
a photon emitted in a process like this, we would see that this packet of light,
this photon, is actually a series of oscillating fields: a magnetic field and an
electric field. Speaking of those fields, sometimes
we abbreviate electric with a capital 𝐸 and magnetic with a capital 𝐵. We can see then that light is an
electromagnetic entity. It’s something that’s made of an
electric field and a magnetic field. This means that the term
electromagnetic radiation is really just a fancy way of saying light. If we understand where light comes
from, we understand where electromagnetic radiation comes from. And it’s that radiation that’s
described in the electromagnetic spectrum.
Now, going back to this emitted
photon, it’s fairly common to represent a photon using a squiggly line like the one
we drew here. The reason for this is that
photons, as we mentioned, have a wavelength associated with them. As it turns out, the wavelength of
photons is one way we can organize the electromagnetic spectrum. This spectrum is the collection of
all electromagnetic radiation possible. At one end of the spectrum, over
here on this side, say, we have light of a very small wavelength. And then as we move from left to
right, the wavelength of light involved gets larger. And notice here that we’re using
the Greek letter 𝜆 to represent wavelength. That’s a common abbreviation.
Recall, we said, that the
electromagnetic spectrum represents all possible wavelengths of light. We’ll show that here using this
wave form that starts out with a short wavelength, and then the wavelength gets
larger and larger as we move left to right. This wave form then is meant to
represent all different wavelengths of light. Now, if we’ve seen a picture of the
electromagnetic spectrum before, we’ve likely seen it divided up into different
regions. There’s one region for visible
light, one for X-rays, one for microwaves, and so on.
While it is helpful to think of
electromagnetic radiation in terms of these sections or regions, it’s important to
keep in mind that physically there is no such separation between different
regions. As far as the spectrum goes, we
observe in nature that light can have virtually any wavelength. But for our purposes, to help us
understand these different wavelengths, we go back in after the fact and divide the
spectrum into different regions.
In general, the spectrum is divided
up into one, two, three, four, five, six, seven regions. And understanding the spectrum
involves being able to recall the name of each one. Knowing the names of these seven
different regions is not as hard as we might think. What we do is start out with one
type of light we’re sure exists. That’s the light our eyes can see,
known as visible light. This is the middle region of the
spectrum. It includes all the colors of the
rainbow, red, green, blue, indigo, violet, and so on.
If we were to look at the colors at
either end of the visible spectrum, on the long-wavelength end of that spectrum, we
would see the color red. And on the short-wavelength end, we
would see the color violet. Knowing these two colors and
knowing to which ends of the visible spectrum they apply is helpful for knowing the
names of the regions on either side of the visible portion of the spectrum.
To understand the names of these
two regions, it’s helpful to know about two prefixes. First, consider the prefix
ultra-. It means beyond. So, for example, an ultramarathon
is a race that’s even longer than a regular marathon. It’s beyond a marathon. As we consider the region of the
spectrum that has shorter wavelengths than visible light, the one just to the left
of it in this sketch, this region gets its name from the fact that it is ultra- or
beyond violet light. Indeed, ultraviolet is the name of
this region of the spectrum. And it’s sometimes abbreviated
capital U capital V.
Examples of ultraviolet waves are
some that are emitted by the Sun. Ultraviolet rays have a smaller
wavelength than visible light, and therefore they transmit more energy. There’s a reliable, inverse
relationship between wavelength and energy. The smaller the wavelength of a
wave is, the more energy it transmits, while a wave with a very large wavelength
transmits very little energy. Because ultraviolet radiation has a
smaller wavelength than visible light, the UV transmits more energy.
Now let’s move on to consider the
region of the spectrum on the other side of the visible portion. This is where our second prefix
infra-, which means below, helps us. Since the visible light at the edge
of the transition between these two regions is colored red, we can name this whole
region below or infrared. And indeed that’s the name of this
whole region of the spectrum. Often, to represent this region,
we’ll see the abbreviation IR, where I stands for infrared and R stands for
radiation.
If we keep going beyond the
infrared region of the spectrum to longer wavelengths and lower energies, we reach
what’s called the microwave region. A good way to recall this name is
to remember that it’s the name of a device we use to heat up our food. Interestingly, microwaves, which we
can use to heat up virtually any kind of food, have wavelengths on the order of 10
to the negative two meters, or one centimeter.
As we move past the microwave
region to the longest-wavelength region of the entire spectrum, we encounter radio
waves. These indeed are just the sort of
waves that are transmitted by radio towers. These waves are at least one meter
in wavelength. Note that there is no upper limit
for the wavelength of a radio wave. A wave with a wavelength that was
tens or hundreds or even thousands of kilometers long would still be called a radio
wave.
As we’ve drawn it, the right side
of our electromagnetic spectrum has larger wavelengths and correspondingly lower
energies. That means, of course, if we travel
in the other direction, we’ll have electromagnetic radiation with shorter
wavelengths and higher energies. If we venture out beyond the
ultraviolet range and go to a higher-energy region, we arrive at what’s known as the
X-ray region of the spectrum. One characteristic of high-energy
radiation like X-rays is its ability to penetrate matter. We’ve probably all had an X-ray
taken of some part of our body, where these waves are high energy enough to transmit
through soft tissue and are only blocked or stopped when they reach something very
dense, like bone. X-ray wavelengths are very small,
about the size of a single atom, 10 to the negative 10 meters.
As we can see though, there’s an
even higher-energy region of the electromagnetic spectrum. The radiation in this part of the
spectrum is called gamma rays. Gamma rays have very short
wavelengths on the order of the diameter of the nucleus of a single atom. The most common source of gamma
radiation is from decaying atomic nuclei. When the nucleus of an atom splits,
breaks apart, often gamma rays are what are released.
If we fill in the approximate
wavelength of ultraviolet, visible, and infrared radiation, then what we have is a
completed electromagnetic spectrum diagram. All seven regions are arranged in
order from lower wavelength and higher energy on the left to larger wavelength and
lower energy on the right. It’s helpful though to add a bit of
information about where these categories of radiation come from. Even though radiation in general
comes from transitioning accelerating electrons, for each of our regions, like we
have for the gamma ray region of the spectrum, we can be more specific about typical
mechanisms for generating the radiation.
A standard way to create X-rays is
to decelerate electrons. This is done by speeding them up
very fast and then slamming them into a stationary target. This is how X-ray tubes generate
X-rays. When it comes to ultraviolet and
visible radiation, the primary source of this light is the Sun. The Sun also creates a good deal of
infrared radiation. It turns out though that infrared
radiation, or IR, is low energy enough that any object in our surroundings is a
source. This radiation is primarily due to
what’s called the thermal motion of atoms and molecules. This is simply the heat-generating
motion of atoms and molecules at regular temperatures, such as room temperature. In other words, any object at room
temperature will emit infrared radiation.
Going on then to microwave and
radio waves, these types of light are created by electric currents, whether
alternating or direct. For both kinds of current, the wave
generation process relies on changes in the current. With alternating current, that
change happens naturally. And for direct current, that change
happens by turning on and off over and over the same direct current. Effectively, this makes DC, or
direct current, behave much like AC, or alternating current.
Now that this chart is complete,
let’s look carefully at the screen and do our best to remember what we see on
it. And now let’s get a bit of recall
practice through an example.
Which of the following could be a
source of infrared radiation? (A) Alternating electric currents,
(B) decaying atomic nuclei, (C) direct electric currents, (D) thermal motion of
atoms and molecules, (E) none of the answers is correct.
We see that each one of the options
(A) through (D) is a candidate for being a source of infrared radiation, a
particular type of radiation in the electromagnetic spectrum. As we consider which of these four
options could be a source of IR, infrared radiation, let’s start out at the top with
option (A), alternating electric currents.
When alternating electric currents
are used to generate electromagnetic radiation, what is typically produced from this
source is either microwaves or radio waves. This is because the frequency of
oscillation of these currents is low enough that it produces these particular types
of radiation. We see that not only option (A)
talks about electric currents but so does option (C), this time in the form of
direct electric currents, that is, current that always points in the same
direction.
Even though direct currents do
always point the same way, we can effectively turn them into alternating currents by
switching the direct current on and off over and over again. It’s by this mechanism that radio
waves are created. What we’re seeing is that both of
these options, alternating as well as direct electric currents, do act as sources of
electromagnetic radiation but not sources for infrared radiation. Instead, they’re typically used to
create microwaves and radio waves. So we’ll cross these off our list
of options.
Moving on to option (B), decaying
atomic nuclei, this is a process where an atomic nucleus breaks or splits apart into
smaller pieces. That’s called fission and, in the
process, releases electromagnetic radiation. When a break like this happens
though, the radiation typically emitted is gamma radiation, that is, the emission of
gamma rays. So, once more, this option is a
source for a particular type of electromagnetic radiation, but not the type we’re
interested in, infrared radiation. We’ll cross off option (B) too.
This brings us to option (D), the
thermal motion of atoms and molecules. Here’s what this option means. Everyday objects, such as chairs or
tables or cups or plants or really anything, will probably be at around room
temperature, that is, about 20 degrees Celsius or 70 degrees Fahrenheit. Just by virtue of their
temperature, these objects will have enough thermal energy that atoms and molecules
that make them up are in thermal motion. Thanks to this thermal motion, a
certain type of radiation is emitted. And this indeed is infrared or
below-red radiation. Our eyes aren’t sensitive to this
particular type of radiation. We can’t see it, but nonetheless
it’s there. And it’s created by the thermal
motion of atoms and molecules.
Answer option (D) can be a source
of infrared radiation. Therefore, option (E), that none of
the answers is correct, is itself not correct. Our final answer is that the
thermal motion of atoms and molecules can be a source of infrared radiation.
Let’s take a moment now to
summarize what we’ve learned about the electromagnetic spectrum. In this lesson, we saw that, in
general, light, which is another name for electromagnetic radiation, is created
through the acceleration of electric charge. Often this acceleration happens in
the context of electron transition between energy levels of an atom.
We saw that the electromagnetic
spectrum organizes all the light that can be produced by the light’s wavelength or,
correspondingly, by its energy. And we also saw that the spectrum
is divided into seven distinct regions. We can arrange the spectrum from
short wavelength, that is, high-energy radiation, to long wavelength, that is,
low-energy radiation. Arranged this way, the seven
regions of the electromagnetic spectrum are gamma rays, X-rays, ultraviolet
radiation, visible light, infrared radiation, microwaves, and radio waves. This is a summary of the
electromagnetic spectrum.
