Video: Eg17S1-Physics-Q29A



Video Transcript

State one effect of increasing the voltage between the anode and the cathode in an electron microscope.

To get started, we can make a rough sketch of just how an electron microscope works. At the start of the imaging process, the electrons that are used in the beam to create an image collect at the tip of a metal cathode. And then thanks to a potential difference that’s set up between the cathode and what’s called the anode, these electrons are accelerated downward. It’s this voltage which accelerates the electrons and gives them a velocity and therefore a kinetic energy.

After they’ve been brought up to speed, the electrons then pass through a series of lenses before they’re incident on the specimen to be imaged. The beam is then reflected to an imaging device. And a picture of the specimen is shown.

Considering this overall process, we know that it takes advantage of the wave nature of electrons. Even though electrons are discrete particles, they also have a wavelength to them. And that wavelength depends on the energy of the electron. Now here’s a question. Say that we had two different electrons and one had a wavelength shown on top and the other had the wavelength shown on bottom. Which electron would you say, based on its wavelength, has more energy?

Well, it’s the one on bottom, right? It’s the one with a shorter wavelength. And there’s another important difference between these waves when it comes to making an image. Imagine that we were to use these two wavelengths to try to image a particular surface that had very small and fine features to it. It turns out that the wave with shorter wavelength is better at showing the very fine details, the small details, in the surface. Another way of saying this is that this shorter-wavelength wave has better resolution or greater resolving power than the longer-wavelength wave.

All this is important for our question because we’re asked what will happen when we increase the voltage between the anode and the cathode in our microscope. We know that, by increasing that voltage, we’ll increase the acceleration of the electrons leaving the cathode. If we increase their acceleration, then by the time they reach the anode, they’ll be moving at a faster speed. If they’re moving at a faster speed, then that means they have more overall kinetic energy, energy of motion. And if they have more energy, then their wavelength will be shorter rather than longer. And as we saw, we can even go a step further and say that if they have shorter wavelength, that means they have better resolving power. That is, they can measure smaller differences in distance to whatever object they’re measuring.

Here then is what we could say about the effect of increasing this voltage between anode and cathode. We know that one effect it has is that it decreases the wavelength associated with the electron beam. That is, it increases the beam energy. And then we also saw that doing that leads to an increase in the resolving or magnifying power of the microscope. Either one of these is an effect of increasing the voltage between anode and cathode.

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