If you look into an electron mirror, you don’t expect to see your reflection. As [Anthony Francis-Jones] points out, what you do see is hard to explain. The key to an electron mirror is that the electric and magnetic fields are 90 degrees apart, and the electrons are 90 degrees from both.
You need a few strange items to make it all work, including an electron gun with a scintillating screen in a low-pressure tube. Once he sets an electric field going, the blue line representing the electrons goes from straight to curved.
The final addition is the magnetic field. A pair of coils do the job. When activated, the magnets deflect the electrons down as opposed to the electric field, which deflects them upwards. The mirror effect is caused when electrons under both forces move downward, seem to strike some invisible mirror, and then move upward again.
Why does it work? [Anthony] explains it very well at the end of the video. If you want to see what the big labs are doing, try trapping electrons. We’ve seen CRTs that use magnetic deflection (usually TVs) and electrostatic deflection (usually oscilloscopes). Other than the screen being the wrong way, it seems like you could do this with a CRT. Those tubes had a long run but are getting harder to find every year.
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