Electron microscopes have got to be my favourite science toys. They are basically bigger and more powerful than normal microscopes that use light because they use beams of electrons. They can allow you to see much tinier features than microscopes that use light; that’s because they are not limited to using glass lenses. The problem with glass lenses is that they often have very slight imperfections that cause aberrations in the image you see when you look down the eyepieces – the image can be slightly blurry in places, especially at high magnification.
Some scanning electron microscopes work by passing a high voltage across a tiny filament of the metal tungsten. The tungsten heats up (to several hundred degrees!) and generates a cloud of electrons. These electrons are guided down a cylinder by magnets and focussed by magnetic lenses into a dense beam where the electrons are travelling at about 30% of the speed of light. The electrons smash into the surface of the sample and either bounce back off (=backscatter electrons) or knock out electrons from the sample itself (=secondary electrons). These electrons are collected by a detector which turns them into an image of the sample surface.
Transmitted electron microscopes are a little different – they use higher energies and the sample has to be really thin – a cross-section – so that the electrons will pass through it.
These microscopes are so much fun to use – it’s amazing to see things at the level of cells and even within cells!