'Microscope' is a very general name given to any apparatus used to inspect very small regions of space 1. Normally, when talking about about microscopes people by default think of 'optical microscopes' which is the most common type of microscopes which are well kown from the biology classes. Many people don't even know that there are other kinds. This entry will present and compare these types of microscopes briefly summing up how they work. For a more detailed information on each type of microscope please refer to the corresponding entries.
Types of Microscopes
Optical microscopes are the most common microscopes, the typical laboratory prop seen in cartoons, known from school, with the unique characteristic hallmark shape of a microscope. These microscopes work like a magnifying-glass. They consist of an intelligent arrangement of optical elements (mostly lenses and mirrors) which lead the light comming from the small observed object directly into the observer's eye or any other light detecting device (such as a film or a digital camera). For that reason these microscopes are also called 'light' microscopes. The optical path of the light, and thus the magnification effect, is characterised by the lenses, and are subject to the laws of optics.
The main advantage of optical microscopes is that they work WYSIWYG, literally. Optical microscopes are non-invasive (that is, they don't exchange matter with the observed object) and are not limited to surfaces of materials. The interior of any transparent body, like cells or crystals can be observed. Optical microscopy is also the oldest form of microscopy, for which reason these microscopes are at a final development stage. They are broadly available and do not require techy gizmos (like piezo scanners or electron beams)to work, not even electric power. The most serious limitation is the resolution limit which is in the best case roughly half the wavelength of the light used,2 and worsened by the quality of the lenses used. The resolution of an optical microscope is typically in the order of 300-400 nanometres.
Electron microscopes use electron beams instead of light and magnets instead of lenses but apart from that the working principle is more or less the same as in the optical microscope, because in a certain way, electrons behave like light. One advantage of using electrons instead of photons (that is: light) is that the electrons have a wavelength that is thousands of times smaller. This means, that the resolution should be thousands of times better than in the optical microscope. In practice a common electron microscope has a resolution of some 500 picometres3.
This tremendous improve in resolution is the main advantage of the electron microscope. The main limitations are: First, electrons cannot readily be routed into the observer's eye, for which reason the whole detection must be steered electronically. Electrons cannot be routed into living organisms without causing undesired side-effects. For that reason it is very difficult to inspect living organisms using electron microscopes. Second, the inspected objects must be electrically conducting or at least be covered with a thin conducting layer, in order to achieve images with best resolution. This requires careful probe preparation. Another serious limitation of electron microscopes is that the observation is restricted to the surface (or at least a very thin layer of some 100 nanometres thickness). Scanning electron microscopes obtain images as a product of electrons that are sort-of 'reflected' by the surface of the object. A transmission electron microscope will only 'shine through' about 100 or so nanometres of material (cutting objects in slices as thin as this is absolutely not an easy procedure).
Scanning probe microscopes. The invention of small piezo motors made it possible to scan tiny needles or tips over very small regions (tens of picometres) of space. If a tip is sensitive and sharp enough to access these regions of space, then they can be used as probes that detect very faint differences of the scanned surface's properties. These differences can be converted into graphic images that look like a topographical map of the studied surface. There are two famous kinds of scanning probe techniques: First, the scanning force microscopy (SFM) which detects the bending of a very sharp tip on a cantilever, thus relaying topological information from the object's surface (it works quite like the old vinyl record-players). Second, the scanning tunnelling microscope (STM) which detects very small currents which in a classic sense shouldn't be flowing between a sharp tip and the object's surface, because they are separated by a very small gap4 (in the order of some nanometres). Additionally to these two techniques there are more, less known and more complex scanning probe techniques, which all have the small scanning piezo-motor in common.
The main advantage of these methods is that they can achieve atomic resolution. That is: Every single atom of a surface can be 'sensed'. Furthermore these needles can be used to plough or scratch the surface creating very small structures, which is interesting for the field of nanoscience. The main disadvantage of the scanning probe methods is their limitation to surfaces (to conducting surfaces in the case of the STM) and the complicated probe-preparation procedure.