Video Transcript
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.
