Video Transcript
In this video, we’re talking about
the electromagnetic spectrum. This spectrum is a start point for
learning about all different kinds of radiation, also known as light. As we’ll see, once we understand
this spectrum, then we’ll also understand lots of the physical phenomena going on
around us.
We can start off on this topic by
considering this question. Where does light come from? Well, for starters, we know that
the Sun creates light, and so does a light bulb, as well as a television or a
computer monitor. Even though there are lots of
different sources of light, there’s a basic physical mechanism 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, often this is 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 an 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. And talking about 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 just a fancy way of saying light, which means if we
understand where light comes from, we know where electromagnetic radiation comes
from. And it’s that radiation which is
described in the electromagnetic spectrum.
Now going back briefly to this
emitted photon, it’s fairly common to represent these photons using a squiggly line,
like we drew here. The reason for this is that
photons, as we mentioned, have a wavelength associated with them. And as it turns out, the
electromagnetic spectrum, which is the spectrum, the collection, of all
electromagnetic radiation possible, that means that, at one end of the spectrum,
over here at this side say, we have light of very short wavelength. And then as we move from left to
right across this spectrum, the wavelength of light involved gets longer and
longer. And notice here that we’re using
the Greek letter 𝜆 to represent wavelength. That’s a common abbreviation.
Now if you’ve seen a picture of the
electromagnetic spectrum before, you’ve likely seen it divided up into different
regions. There’s one region for visible
light, one region for X-rays, one region from microwaves, and so on and so
forth. While it is helpful to think of
light of electromagnetic radiation according to these regions or sections, it’s
important to realize that physically there is no such separation between different
regions.
If we go back to our sketch of the
spectrum that we saw on the opening screen, we see that there’s no division between
different regions of the spectrum. The wavelengths just smoothly and
continuously get longer as we move from left to right. So 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 up
the spectrum into different regions.
In general, the spectrum is divided
up into one, two, three, four, five, six, seven different regions. And understanding the spectrum
involves being able to recall the name for each one. Knowing these seven names is not as
hard as we might think. What we do is we start out with a
one type of light we’re sure exists. That’s the light our eyes can see,
also known as visible light. This region is at the very center
of the spectrum. And it includes all the colors of
the rainbow: red, green, blue, violet, et cetera.
If we were to look at the visible
colors on either end at the extremes of the visible spectrum, on the long-wavelength
end of the spectrum, we would see the color red. And on the short-wavelength end, we
would see the color violet. Knowing these two colors and
recalling which ends of the visible part of the spectrum they apply to is helpful
for knowing the names of the regions on either side of the visible part.
To recall the names of these two
regions, it’s helpful to know a little bit about a couple of different prefixes. First, consider the prefix
“ultra.” It means beyond. So, for example, an ultramarathon
is a marathon 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. And indeed, ultraviolet is the name
of this region of the spectrum. And it’s sometimes abbreviated
capital U capital V. These are the types of rays created
by the Sun, which are higher energy than visible light. And indeed, that comes from the
fact that they have a shorter wavelength. We can see that they’re farther to
the left on our spectrum than visible radiation. So let’s add that to our
description of what either end of our electromagnetic spectrum involves.
On the left end, we have light with
a relatively short wavelength and therefore a higher energy. While on the right end, we have
light with longer wavelength and therefore lower energy. So the higher-energy region of the
spectrum that’s adjacent to, right next to, visible radiation is called the beyond
violet or ultraviolet region.
But then what about the other side
of the visible part of the spectrum? This is where our second prefix,
“infra,” which means below, helps us. Since the visible light that’s at
the edge of this transition between the two regions is colored red, we can name this
whole region below or infrared. And indeed, that’s the name of this
region of the spectrum, infrared. And often, to represent this
region, we’ll see the abbreviation IR, which stands for infrared radiation.
If we keep going beyond the
infrared part of the spectrum to longer wavelengths and even lower-energy waves,
what we’ve hit on is the microwave region of the electromagnetic spectrum. A good way to recall this name is
to realize it’s the name of a device we often use to heat up our food, our
microwave. Interestingly, these waves, by
which we heat up virtually any type of food, have wavelengths on the order of 10 to
the negative two meters or one one hundredth of a meter long.
Then as we move past the microwave
region to the longest-wavelength region of the whole spectrum, we encounter the
range of radio waves. These indeed are just the sort of
waves that are transmitted by radio towers. Using a radio selector, we tune in
to different channels of radio wave radiation. These waves are at least a meter
long and can be much longer in wavelength than that.
Now as we said, the far right edge
of the spectrum has longer wavelengths and correspondingly lower energies to these
waves. That means of course that if we
travel in the other direction, we’ll have shorter wavelengths and higher-energy
electromagnetic radiation.
If we venture out beyond the
ultraviolet range and go to a higher-energy region, then we’ve arrived 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 through 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,
on the order of the size of an atom, 10 to the negative 10th meters.
But as we can see, there’s an even
higher-energy region of the electromagnetic spectrum. The radiation in this part of the
spectrum is known as gamma rays. Gamma rays have very short
wavelengths, less than 10 to the negative 15th meters. And their most common source is
decaying atomic nuclei. When the nucleus of a decaying atom
splits, when it breaks apart, often times gamma rays are emitted.
If we fill in the approximate
wavelengths of ultraviolet, visible, and infrared radiation, then what we have is a
completed electromagnetic spectrum diagram. We have all seven regions arranged
in order, left to right, from shorter wavelength to longer wavelength or,
correspondingly, from high energy to low energy. This is the spectrum as we’ll often
see it.
But it’s also helpful to add a bit
of information about where these different types of radiation come from. Even though radiation in general
comes from transitioning accelerating electrons, for each of these regions, we can
get more specific about the typical mechanism that generates this radiation. If we start at the left side of our
spectrum, with gamma rays, the highest-energy radiation, as we said, these are
typically generated through nuclear decay, that is, the radioactive deterioration of
atomic nuclei.
A standard way to create X-rays is
to rapidly decelerate electrons by speeding them up very fast and then having them
slam into a stationary target. This is the general mechanism by
which 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. But it turns out that infrared
radiation, or IR, is low energy enough that any object in our surroundings is a
source. That’s because this radiation is
due to what’s called the thermal motion of atoms and molecules. In other words, just by virtue of
being at, say, room temperature, an object will emit infrared radiation.
And then going beyond infrared
radiation to microwave and radio waves, this type of light is created by electric
currents, whether alternating or direct. For both kinds of current, the wave
generation process relies on changes to the current. With alternating current, that
change happens naturally. And for direct current, the change
happens by turning on and then off over and over again that same direct current. Effectively then, this makes the DC
or direct current behave a lot 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 up 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, but this time in the form of direct
electric currents, that is, currents that always move in the same direction.
Even though direct currents do
always move the same way, we can effectively turn them into alternating currents by
switching these direct currents on and off over and over again. It’s by this mechanism that radio
waves can be created. What we’re seeing is that both of
these options, alternating as well as direct electric currents, do act as sources
for electromagnetic radiation, but not sources for infrared radiation. Instead, they’re typically used to
create microwaves and radio waves. So we’ll cross those off our list
of options.
Moving on to option B, decaying
atomic nuclei, this is a process where an atomic nucleus splits or breaks apart into
smaller pieces — that’s called fission — and in the process releases energy through
electromagnetic radiation. But the type of radiation typically
emitted through this process 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. So we cross option B off our list
too.
Next, we get to option D, the
thermal motion of atoms and molecules. Here’s what this option means. Everyday objects such as chairs,
tables, and the like, simply by being at room temperature, about 20 degrees Celsius
or about 70 degrees Fahrenheit, has enough thermal energy that the atoms and
molecules in these objects are in thermal motion. And thanks to this thermal motion,
a particular type of radiation is emitted. And this indeed is infrared or
below-red radiation. That is, our eyes aren’t sensitive
to this particular wavelength of radiation. But nonetheless, it’s there and
it’s created by thermal motion of atoms and molecules. This option can be a source for
infrared radiation. And therefore, option E that none
of the answers is correct is itself not correct. And so our final answer is that
thermal motion of atoms and molecules could be a source for 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. And that often this acceleration
happens in the context of electron transition within energy levels of an atom. We saw that the electromagnetic
spectrum organizes all the light that can be produced by the light’s energy or
equivalently by its wavelength. And we also saw that the spectrum
is divided into seven distinct regions. If we arranged the spectrum from
high-energy radiation, that is, radiation with shorter wavelengths, on one end, and
low-energy radiation, radiation with longer wavelengths, on the other. Then going from high energy to low
energy, these seven regions are gamma rays, X-rays, ultraviolet radiation, visible
light, infrared radiation, microwaves, and then radio waves.
