The first simple "microscopes" usually only magnified objects by around 6x. They consisted of only one lense and were what we would call magnifying glasses. During the late 16th century it was discovered that a combination of two lenses led to a much greater magnification.
Microscope technology devloped and in the late 17th century Antony Van Leeuwenhoek became the first person to produce lenses of a quality suitable for more in depth magnification. Robert Hooke is credited with the microscopic milestone of discovering the basic unit of all life, the cell.
The Light microscope uses a beam of light to form an image of an object and it is limited in it's magnification to around x2000. In the 1930's the invention of the electron microscope allowed scientists to observe with grater detail the sub-cellular structures within a cell. These microscopes use electron beams to magnify images up to x2,000,000.
There are two types of electron microscope:
The Scanning Electron Microscope (SEM) produces 2-dimensional images with very high magnification and resolution
The Transmission Electron Microscope (TEM) produces 3-dimensional images but at lower magnifications.
When using a microscope it is important not only that it can magnify an image, but also that you can determine the difference between two objects in the viewfinder. These are two key different terms:
A light microscope has a resolving power of about 200nm, whilst an electron microscope can have a resolving power of up to 0.2nm!