Video: Electromagnetic Waves and Matter

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

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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.

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