ABSTRACT
Microscopes are devices that generate magnified images of small objects such as cells and sub-cellular structures by using optical principles, streams of charged particles or scanning probes. Electron microscopy is an example of charged particle microscopy. Instead of a light source, a stream of accelerated electrons is used for imaging. The strong interaction of electrons with matter enables imaging of single molecules. In fluorescence microscopy, a type of light microscopy, fluorescence emitted by molecules in the sample is used to generate images of tissues, cells, and sub-cellular structures. Optical light paths are reversible, meaning that light emitted from a point source in the focal plane of a focusing lens will end up as a parallel light bundle on the opposite side of the lens, which is called collimation. Strictly speaking, a lens can only focus light into a focal point in the absence of diffraction.
