Why do we need microscopes?
We use microscopes to look at small things. With your eyes, you can (if you are young and have healthy eyes) see things as small as 1 mm (1/1000 of a metre), or maybe even a little smaller. But many interesting things, or interesting parts of things, are even smaller than that. To be able to look at those, our eyes need help! For a little help, to look at things that are just too small to see, we can use a magnifying glass. These are excellent for looking at mini-beasts from a pond, or details in a flower. They are easy to carry and they make you look like a detective!
But if you want to look closer, you may need to use a microscope. Microscope literally means "small-seer" and they come in many sizes. Usually, the smaller the things you are looking at, the bigger your microscope needs to be! Some microscopes also use lasers to be able to look at specific parts of the sample. We like looking at really small things, so we use the biggest microscopes.
Why use electron microscopes?
Magnifying glasses and microscopes are just helpers for your eyes. They still use the same light we normally see with. But some things are too small to see with light. How does that happen?
Look at the left-hand picture below. I took it in the garden. There is an animal in it, can you see? How about if I make it bigger, on the right?
You may have guessed there is a cat there, or possibly a rock that looks like one. The bigger picture on the right got bigger, but it didn't get clearer, it got blurry. Something similar happens when you try to look at really small things in a microscope - they get blurry. So we need to use something other than light: electrons!
The electrons are made at the top of the microscope, by passing electricity through a crystal. We then use magnets to get the electrons to the sample, and more magnets to get the electrons from the sample to the detector. Our eyes can't see electrons, so we have to change them into something we can see. This is done by catching the electrons on a layer of chemicals that make light when the electron hits them. We also have a camera connected to the computer so we can save the images. Inside the electron microscope there is a vacuum, otherwise the electrons would be slowed down too much. The electrons travel at a third of the speed of light!
Most of the things we want to look at, such as viruses or cells, live in water and are made mostly of water. If you put them in a vacuum, they explode, and if you hit them with a beam of electrons, they burn. Also, the electrons make an image is by bouncing off the sample, and the electrons don't bounce very well off the stuff that our samples are made of. To be able to see them properly, we need to treat them. To prepare a virus sample, we put a drop of the sample on a very thin plastic film and cover it in a heavy metal salt. What we see in the microscope is dark patches where the heavy metal is, and lighter patches where there were virus particles.