Video Transcript
In this video, we’re talking about
the interactions of electromagnetic waves with matter. These interactions determine most
of what we see with our eyes when we look out at the world. And they also help us understand
what will happen when we shine light of a particular wavelength onto some certain
material. Now, in order to understand these
interactions, we’re going to need to know about the two parts involved,
electromagnetic waves and matter.
Let’s consider those one at a time,
starting out with the waves. Electromagnetic waves, sometimes
called EM waves for short, come in a variety of wavelengths. For example, of these three waves,
the one on top has the longest wavelength. And then, the wavelength gets
shorter as we go down. The wavelength of an EM wave is
important because it tells us how much energy that wave possesses. Here’s how these two things are
related. The smaller or the larger the
wavelength of the wave, the more or the less energy the wave carries. This means that if a wave has a
small wavelength, then it carries a lot of energy. But if it has a large or long
wavelength, it has less energy.
So, looking again at our three
waves, the one on top has the largest wavelength. But that actually means it carries
the least energy. The one on bottom with the shortest
wavelength has the most. In general, an electromagnetic wave
can have any wavelength. There’s really no upper or lower
limit. There are waves, for instance, that
exist with wavelengths longer than the wavelength of our pink wave. And there are waves that have
wavelengths in between the pink and the green, and between the green and the blue,
and waves that have wavelengths that are shorter than the blue.
All these different waves, with all
these different wavelengths have been organized into what’s often called the
electromagnetic spectrum. This spectrum puts waves with a
very small wavelength at one end. And then, as wavelength increases,
the ones with a large wavelength go at the other end. It’s basically a way of putting
electromagnetic waves in an order according to their wavelength. As we draw out our own version of
the electromagnetic spectrum, let’s put the short-wavelength waves on the left side
and the long-wavelength waves on the right.
And this little sketch here at the
top shows us that wavelength increases as we go from left to right. Knowing all this, we can now sketch
in the seven distinct regions of the electromagnetic spectrum. Here are those seven sections. First, gamma rays that have the
highest energy and the shortest wavelength. Then, up to X-rays with longer
wavelength and less energy, then ultraviolet radiation, visible light, the light our
eyes can see, infrared radiation, called IR for short, then onto microwaves, and
finally radio waves, the waves with the longest wavelength and the least energy.
Now, we’ve seen that the wavelength
of this radiation goes up as we travel from left to right. And based on what we saw earlier
about the relationship between wavelength and wave energy, we know that if
wavelength increases as we go from left to right, that means that energy decreases
as we move that same direction. Or another way to say it is that
the energy increases as we go from right to left, as we go towards shorter
wavelengths. This means, for example, that
infrared radiation is more energetic than microwaves, and that visible light is more
energetic than infrared, ultraviolet more energetic than visible, and so on.
And it’s at this point that matter,
the stuff that these waves interact with, comes into play. We know that, in general, matter is
made up of atoms. Atoms, we can recall, are made up
of a core called a nucleus, where the nucleus consists of two different types of
particles. Neutrons, what we’ve colored here
in green, and protons, charged particles that we’ve colored here in blue. And we know that there’s a third
type of particle that helps to make up atoms. These are called electrons. They’re negatively charged
particles that rotate, or orbit, around the nucleus.
And it turns out that these
electrons can be closer to or farther away from the nucleus, depending on how much
energy each individual electron has. For an electron with a very low
amount of energy, it would orbit the nucleus very close by. But let’s say that one of these
three electrons has a higher energy level than the other two. Well, in that case, that higher
energy electron would orbit the nucleus a greater distance away. We would say this electron has a
higher energy level than the other two.
Now, that doesn’t mean that this
arrangement is fixed for all time. It is possible for electrons to
move back and forth in-between energy levels. This movement often happens thanks
to an interaction with an electromagnetic wave. Let’s say that we have this
situation. Our atom is right over here. And an electromagnetic wave comes
to interact with it. When this happens, one of three
things can take place. The first possibility is that our
wave is transmitted through the atom. In other words, it just passes
right through. An example of this is visible light
passing through window glass. The light passes right through the
glass so our eyes can see it. It’s transmitted.
The next possibility is that our
wave is reflected. We know there are some materials,
like the glass in a mirror, that are specifically designed to do this, to reflect
light. But then, there’s also this third
possibility that our electromagnetic wave is absorbed by this atom. When we consider the interactions
between EM waves and matter, often it’s this option, absorption, that we’re talking
about. Here’s how absorption of this
electromagnetic energy that’s carried by this wave can work.
We talked earlier about the
wavelength of electromagnetic waves and how that helps us determine the energy
carried by the wave. That energy amount is very
important. The reason is that for an atom to
absorb some electromagnetic wave, in other words, for an electron in the atom to
absorb a wave, the energy that the wave carries has to be a match for the energy
that the electron can accept. In general, an electron can only
accept an amount of energy that lets it move up to another energy level. So, if there isn’t a match in those
amounts of energy, the energy in the wave and the energy the electron can accept,
then the wave would just pass right through. It would be transmitted.
But if there is a match, the
electron is able to absorb the energy in the wave, and then it moves up to a higher
energy level. Now, this explains how electrons
can move from lower to higher energy levels. But what about the electrons that
are already at the highest possible energy in this atom? What if they absorb an EM wave? It turns out that this can
happen. And when it does, the electron then
has so much energy that it effectively escapes from the nucleus. It’s a bit like if a satellite was
orbiting the Earth, and it kept speeding up faster and faster and faster. Well, eventually, it would speed up
so fast that it would escape Earth’s gravitational pull.
It’s not gravity that keeps
electrons in orbit around the nucleus, but the idea is similar. This electron now has so much
energy that it escapes from the atom entirely. At this point, let’s recall a
certain scientific term. That term is ion. And ion is a charged particle, or
it’s a particle that has a net electric charge. As we think about our atom here,
originally, this atom was not an ion. That’s because it had the same
number of positive charges, charges in the nucleus, as it had negative charges,
electrons.
But now, look at what’s
happened. We’ve lost an electron, which has a
negative charge, which means that the atom left over now does have an imbalance of
electrical charge. There are now more positive charges
than negative charges. And so, this atom has become an
ion. We could say that it has been
ionized. For an atom to be ionized, for an
electron to be ejected from it, it takes a lot of energy from an incoming
electromagnetic wave. In fact, as we look again at our
electromagnetic spectrum, only certain categories of waves on this spectrum are
capable of ionizing atoms.
That is, only some of these classes
of radiation are high-energy enough to strip an electron off of an atom. Now, if we were to draw a line
showing the energy required to ionize or remove an electron from an atom, that line
falls somewhere within the ultraviolet span of the spectrum. For any waves in the spectrum that
have a shorter wavelength, and therefore higher energy than this, those waves are
called ionizing radiation. And we can see that they include
X-rays and gamma rays and some ultraviolet rays.
What we’re saying, then, is that
all gamma rays, all gamma radiation, has enough energy to strip an electron off of
an atom. The same is true for X-rays. All X-rays are capable of this as
well. And then, when we come to slightly
lower energy ultraviolet radiation, certain wavelengths of ultraviolet light can do
this and certain don’t have enough energy to. This means that ultraviolet
radiation may be ionizing or it may not be. It depends on the specific
wavelength. The reason we’re so interested in
whether radiation is ionizing or not ionizing is because this phenomenon can have a
significant impact on human health.
Think about the cells in our
body. Those cells, of course, are made of
atoms. Those atoms start off electrically
neutral, but if they’re exposed to ionizing radiation, then they become ions. They become charged particles. When this happens for many of the
atoms in a cell, there are three different ways the cell can respond. Number one, the cell can die. This ionization damage can be so
great that the cell isn’t able to divide anymore. Or number two, the cell is able to
repair itself and go on dividing as usual.
And finally, it’s possible for the
cell to mutate, to experience some unintended change in its genetic code. This is perhaps the most dangerous
of the three possible outcomes because it can mean that the cell begins to
replicate, or reproduce, unhealthy cells. And after enough dysfunctional
replication, a tumor can develop. So, ionizing radiation is
considered dangerous to our cells. And therefore, we avoid it wherever
possible. This is one reason, for example,
that sunscreen is to be worn whenever we’re out in direct sunlight for a long
time. It’s to protect our skin from the
ionizing UV-radiation given off by the sun.
Now, of all the waves on our EM
spectrum, we can see that gamma rays have the highest energy, that is the shortest
wavelength. And this means that of all the
possible kinds of waves that could run into an electron that’s already at the outer
energy level, gamma rays are the waves most likely to cause that electron to leave
the atom entirely. That’s because they deliver the
most energy of any wave. From the perspective of creating
ions then, we could say that gamma rays interact with matter more strongly than any
other kind of wave.
And when we say that, we mean that
these rays have the most energy to deliver to an electron. Like we saw earlier though,
ionizing an atom by stripping an electron away is not the only or even the dominant
way that electromagnetic waves interact with matter. It’s more common for
electromagnetic waves to cause electrons in the atoms to move up to higher energy
levels within the atom. And we saw that there needed to be
a match between the energy carried by that particular wave and the energy difference
between where the electron was and where it would end up.
This tells us that different types
of waves — visible light, infrared radiation, microwaves — will be easier to absorb
by some materials and harder to absorb by others. In other words, the type of wave
that an object absorbs most easily depends on the object. For example, consider putting a
bowl of food in a microwave in order to heat it up. When we press start on the
microwave, the microwave radiation reaches the food as well as the bowl. But because the particular energy
level of these waves is a good match for heating up water, the food, which has water
molecules in it, tends to heat up more readily than the bowl, which doesn’t have
much water.
The food is able to absorb the
energy carried by these waves better than the bowl can. And that’s because the energy
carried by these microwaves is a better match for the possible energy transitions in
the food molecules than they are for the possible transitions in the molecules that
make up the bowl. All this to say, depending on what
type of matter we’re considering, different kinds of waves — whether radio waves, or
infrared radiation, or X-rays — will interact more or less strongly with that
matter. Now, to check our understanding of
what’s happened so far, let’s look at an example exercise.
One or more of the following types
of electromagnetic radiation have no ionizing wavelengths. Which of these types of radiation
cannot be ionizing? a) Ultraviolet. b) Radio. c) Infrared. d) X-ray.
Okay, the first thing we can look
at is what is ionizing radiation? What does it mean for radiation to
be ionizing? Let’s say we have an atom. And in the core of our atom, we
have just as many protons, which have a positive charge, as we do electrons, which
have a negative charge, orbiting that nucleus. So, our atom has just as many
positive charges as it has negative charges. It has no net charge. Therefore, it’s not a charged
particle, and that’s what an ion is. A particle with a net charge.
In order to ionize an atom to give
it a net charge, the most common method is by stripping away one of its higher
energy electrons. And this is where these different
types of radiation get involved. If an electromagnetic wave with
enough energy is absorbed by one of these high energy electrons, then that electron
can gain so much energy by the absorption that it’s effectively ejected from this
atom. Once the electron leaves an atom,
that means the atom left behind now does have an overall, or net, electric
charge. In other words, it’s an ion. And that ion was created through
this absorption process.
Now, the ability of this incoming
wave to cause an electron to be ejected from the atom all depends on how much energy
the wave carries. There’s some minimum amount of wave
energy required in order to do this. This means that not all radiation
is ionizing radiation. Not all waves have enough energy to
do this. In this exercise, we wanna pick out
which of the types of radiation from our list cannot be ionizing, in other words do
not have enough energy to eject an electron from an atom.
To see which radiation types those
might be, let’s briefly recall the seven categories of radiation which were
summarized in the electromagnetic spectrum. If we organized our spectrum so
that higher wavelengths were off to the right and higher energies were off to the
left, then our seven categories would be ordered like this. Gamma rays on the far left, then
X-rays, then ultraviolet light, visible light, the light our eyes can see, then
infrared radiation, microwaves, and then finally radio waves with the longest
wavelengths of them all.
We noted earlier that it’s the
higher energy waves which are capable of ionizing an atom. And it turns out that the cut off
energy, the minimum energy required to do this, occurs in the UV part of the
spectrum. In other words, for some
ultraviolet radiation and for all X-rays and all gamma rays, that radiation is
energetic enough to be ionizing radiation. But then, on the other side of this
line, the radiation doesn’t have enough energy to strip an electron off of an
atom. It can’t ionize an atom. So, now, let’s look at our four
options a, b, c, and d.
Option a, ultraviolet is a bit
interesting because we see that this is the region of the spectrum where the energy
cut off occurs. But look now at the specific
wording of our question. Which of these types of radiation
cannot be ionizing? Because there are some wavelengths
of ultraviolet light which are ionizing, we can’t choose a as one of our
answers.
Look, then at option b, radio
waves. We see these waves are all the way
to the right of our spectrum. They have the highest wave length
and the lowest energy. These waves are far to the right of
the ionization cut off. So, these cannot be ionizing. And similarly with infrared
radiation, which again is to the right of that energy minimum. Our last option, X-rays, indicates
wavelengths of light which are all high energy enough to be ionizing. Option d, then, is not an answer
that we’ll choose. So, of these options, it’s only
radio waves and infrared radiation which cannot be ionizing.
Let’s summarize now what we’ve
learned in this lesson. Starting off, we saw that
electromagnetic, or EM, waves carry energy. And the smaller their wavelength
is, the more energy these waves possess. Then, we talked about matter. And we saw that it’s made of atomic
nuclei as well as electrons with different energy levels that orbit the nuclei. We learned that electromagnetic
waves can be absorbed by electrons, which raises the electron’s energy level.
If the wave has enough energy, then
an electron can leave an atom entirely, creating a charged particle, an ion. The waves that have enough energy
to do this are gamma rays, X-rays, and some wavelengths of ultraviolet waves. These are ionizing radiation. And finally, we saw that the type
of radiation an object interacts with most easily depends on the energy levels in
that object’s atoms.