The Light Microscope
The most common, inexpensive, and easy to use microscope is the light microscope, which produces a magnified image of the object by bending and focusing light rays. The light microscope uses a variety of glass lenses to produce a magnified image that is focused before the eye. The magnifying properties of a converging lens, like that which is used in a typical magnifying glass or camera. Light from the object is bent, or refracted, as it passes through the lens producing an image which is inverted and magnified. In the simplest compound microscope, two converging lenses are used. The image from the first lens (objective) becomes the object for the second lens (eyepiece). The final image is much larger than either lens could produce independently. With a little effort, you can reproduce this effect yourself by using two magnifying glasses.
The wavelength of visible light ultimately limits the resolving power of the light microscope. Therefore, two objects separated by distances significantly less than about 0.4 micrometers (the smallest wavelength of visible light) cannot be distinguished as separate. This is because the light microscope produces its images by reflecting from or transmitting visible light through a specimen. An analogy can be made to ocean waves at the beach, with wavelengths of a few meters. If two people were wading into the surf only a few inches apart (a separation much less than the wavelength of the ocean waves), it would be impossible to distinguish them as separate by analyzing the ocean waves that reflected from them. Despite these limitations, the resolution of the light microscope is sufficient to produce excellent images of many of the important cell structures and organelles, and consequently still has many applications, chiefly in biology.
Science EncyclopediaScience & Philosophy: Methane to Molecular clockMicroscopy - The Light Microscope, History Of Light Microscopy, Electron Microscopy, Scanning Tunneling Microscopy, Recent Developments In Microscopy