Microscopes give us a large image of a tiny object. The microscope works a lot like a telescope except that the object is very close to the objective lens.
A microscope is basically a two-part magnifying glass. Most of the time, when we refer to a microscope, we mean a compound microscope or an objective and an eyepiece mounted in a tube. The objective is the part that you can think of as a magnifying glass, or lens. The eyepiece is also kind of like a magnifying glass, but smaller, and serves to direct the light straight into your eye. In a typical compound microscope, these two lenses are mounted on either end of a tube like this:
… where the eyepiece is located on the "eye end" of the figure and the objective is located at the "object end" (see the curved lens held in place by two black holders towards the object end of the tube?). That's really all there is to it! The objective lens focuses a magnified image of the object at exactly the focal length of the eyepiece lens. The magnified image is then directed by the eyepiece straight into your eye.
Let's put a tiny organism (a "daphnia" or water flea) we found in pond water on a glass slide. The clips on the microscope's flat stage hold the slide in place. Now take a look through the microscope eyepiece. A mirror at the bottom of the microscope reflects light rays up to the daphnia through a hole in the stage. Objective lenses magnify the image which is made even larger when we see it through the eyepiece lenses.
The number marked on the objective lens tells us how many times the organism we have on the slide is being magnified. It says 40x. Wow, that means the tiny daphnia is forty times larger than life. To focus we can move the objective lens closer to or farther from the organism.
The objective lens is usually a compound lens, a combination of two lenses made from different kinds of glass. When only one lens is used, we often get distortion. This distortion (chromatic aberration) is caused because the colours making up light are not refracted (bent) the same amount when passing through a glass lens. When we use a compound lens, any distortion from the first lens is corrected by the second lens.
Electron microscopes use beams of electrons instead of light. The electron beam is moved around using magnets which act like the lenses in an ordinary microscope. Electron microscopes can magnify objects over 200,000 times. Using a scanning tunneling microscope (STM), scientists have actually been able to look at atoms, minute units of matter. The computerized STM moves above a material. Electrons jump from the tip of the STM to the surface of the material being scanned. This creates an electric charge. The electric current changes as the tip is moved across the material. By moving the tip back and forth across the material, the computer can map the electron clouds (and the atoms).
Different types of microscopes have been used to look at human cells, identify minerals, solve crimes, see how freezing affects food, study metals, and find the causes of crop diseases. Microscopes are an essential tool in medicine too. They have been used to identify the causes of many deadly diseases like malaria and tuberculosis. Microscopes can also help to find out why a person or animal died.
Scientists can even use a microscope to figure out where illegal drugs come from. For example, looking at opium crystals through a microscope reveals different shapes depending on where the poppies they came from were grown. This information can help pinpoint the source of illegal drugs.
You can do all kinds of amazing things with a microscope. The possibilities for what you can find out with a microscope are endless. And who knows you might identify something new and it may even get named after you!