Transcription
In the previous talk, we looked at the electromagnetic force and how the movement of currents or the flow of electricity could induce magnetism, and how the movement of magnetism could induce current. That leads us nicely to looking at electromagnetic radiation and the electromagnetic spectrum, as we segue now into our X-ray Physics course proper.
So, the electromagnetic spectrum can be drawn in many different ways, and I like to orientate it vertically like this. I find it much easier to understand. Now, you'll see our electromagnetic radiation or our electromagnetic wave through the center of this diagram. Now, as we head up on this diagram, we see the wavelength, the distance between successive peaks in the wave, increases. As we head up, not only does the wavelength increase, as we head up, but our frequency, or the number of cycles of waves per second, decreases. As we head at this diagram, as we had lowered down in the diagram, our frequency increases, the number of waves passing through a particular point over a period of time increases, and our wavelength decreases. We can see they're inversely proportional to one another.
Now, the wave itself is exactly the same. The structure of the wave is the same. Only the wavelength and frequency is changing here. Now, we can divide this spectrum into multiple different categories depending on the wavelength of these waves. You'll see in the middle of this spectrum is visible light, running from about 700 nanometers to just below 400 nanometers. We've got violet extending all the way up to red here. Now, this is the most common electromagnetic frequency within our universe, and our eyes have adapted to see this electromagnetic radiation, and what we call visible light.
Now, luckily, above the red part of the spectrum, we've named that infrared. These wavelengths are in the region of micrometers. As that wavelength gets longer, into the region of centimeters, we call those microwaves. And as it gets longer, into the region of meters, we call those radio waves. If we go below the violet part of the spectrum, we now call that ultraviolet light. Not visible light to our eyes. The frequency has increased, our wavelength has decreased. As we then increase our frequency further, we then classify those waves as X-rays. And as that wavelength gets slightly shorter, we call that gamma rays.
Now, as we're going to look at, as frequency increases, the energy of that wave increased. And in our previous talk, we were looking at binding energy of an electron within an atom. And binding energy was the energy required to release that electron from an atom. And the release of an electron from an atom is what's known as ionization. We've ionized that atom. And it turns out that X-rays and gamma rays, their frequencies are high enough, their energies are high enough to release electrons from atoms. And we call this ionizing radiation. Visible light and up does not have enough energy to ionize atoms.
So, let's have a look at the construction of an electromagnetic wave and get a couple of definitions out the way first. We can see this wave is traveling along our x-axis here through space. And we can see the wave is perpendicular to the direction of its flow. Now, the distance between successive peaks in the wave is what's known as the wavelength. We generally measure that in meters. Our wavelength, the number of waves that pass a particular point over a period of time, is known as the frequency of the wave. And our amplitude of the wave here represents the intensity of that wave. And generally, when we're talking about electromagnetic radiation, the amplitude is proportional to the number of photons in that electromagnetic radiation.
Now, as I've said, radio waves, microwaves, X-rays, and gamma rays, the wave itself, the construction of the wave is exactly the same. The only thing that changes is the wavelength and the frequency. Now, electromagnetic waves are self-propagating waves, as I'm going to show you now. And they travel through a vacuum at a constant speed, and that's the speed of light. It doesn't require a medium to propagate itself. Propagates. So this light can travel in a vacuum. So radio waves and X-rays travel at the same speed. And our velocity here is exactly the same for all electromagnetic radiation going through a vacuum. This is the speed of light. So if our velocity is staying the same, our frequency and our wavelength need to be inversely proportional. As our frequency increases, our wavelength needs to decrease in order to keep our velocity the same.
Now, electromagnetic waves are what is known as transverse waves. That's because the movement of energy, or the direction of the wave, is perpendicular to the movement of those waves. And if you see how this wave travels a long time in space, and we take a particular point in that wave and watch what that point does, that point will go up and down like this. So let's have a look at this now. We can see that as the wave travels, this point will go up and back down, perpendicular to the movement of that wave. You see how it's going like that? Now, we can get longitudinal waves where the molecules move in the same direction as the propagation of the wave. And we're going to look at that in closer depth when we look at our ultrasound waves. But the movement here in electromagnetic radiation is transverse to the movement of the wave.
Now, in fact, electromagnetic radiation is not as simple as I've shown you there. It's in fact two transverse waves that are orthogonal to one another. They lie at 90 degrees to one another. And this is where our concepts from the electromagnetic force come into play. I've shown you before that the movement of current, the movement of the electric wave here, represented in yellow, induces magnetism, induces this magnetic wave here, represented in blue. The movement of magnetism induces current. This is why this is a self-propagating wave. Electromagnetic radiation can travel without the need for a medium. When we look at ultrasound, we'll see that a medium is required in order for an ultrasound wave to propagate. But because this photon is self-propagating, this does not require a medium to travel.
Now, electromagnetic radiation gets further complicated. Not only is it two orthogonal transverse waves, but we can actually think of electromagnetic radiation as a particle. It acts both as a wave and as a particle. And this is what's known as wave-particle duality. Now, electromagnetic waves act as waves. They diffract, they interfere with one another. But there are certain set of experiments that show these waves also acting as particles.
Now, throughout this X-ray module, we're going to look at a concept called the photoelectric effect. And it was actually the photoelectric effect that showed these waves acting as particles. What scientists did was they took a sheet of metal and they shone visible light on their sheet of metal. Now, if electromagnetic waves were to act purely as waves, we would expect as we increase the intensity of that light, we increase the amplitude of that wave, we would think the energy would go up and there would be enough energy to release an electron from that metal. And when scientists did that, there was no release of electron. What they noticed was when they increase the frequency into the range of X-rays, even the smallest intensity of X-ray would release an electron. And it showed now these waves acting as particles, as discrete packets of energy and mass. And it turns out, the world is quantized. Electromagnetic radiation can both act as a wave as well as a particle.
And when we are calculating the energy of a wave here, we can see that the energy of the wave is proportional to the wave's frequency. As the frequency of that wave increases, so does the energy. But you can see that the amplitude of the wave is not included in this equation. It doesn't matter the intensity of the light, the number of photons of light doesn't increase the wave's inherent energy. You may notice here that we are also multiplying by this letter H, and this is known as Planck's constant. So you can see that energy is not only proportional to frequency, but it's also a multiple of a constant. And the fact that it's a multiple of a constant speaks to the way that energy is also quantized. So electromagnetic radiation can act both as a wave and as a particle. And this is known as wave-particle duality.
So, it's really important to understand electromagnetic radiation, especially when looking at X-ray physics. After all, X-rays are electromagnetic radiation. So this is a good place to start our X-ray physics module. And if you want to get access to a question bank of X-ray physics related questions that have come up in past papers, see the first line of the description below. I've curated a bunch of X-ray physics questions for you that have come up in multiple different exams over the last 10 years. So check it out if you want. Otherwise, I'll see you for our overview of X-ray physics in the next talk. Goodbye.