Microscopes work by gathering light and focusing it through a series of lenses to create an enlarged image. Optical microscopes use a light source to illuminate a specimen and utilize condenser and ocular lenses to provide magnification.
The most basic microscopes utilize a single lens, essentially a magnifying glass, to produce a relatively high level of magnification. Optical microscopes are similar to refracting telescopes and operate using many of the same basic optical principles. While telescopes are designed to gather light from distant objects, microscopes make use of a smaller viewing area and an independent light source to illuminate and magnify a specimen. The objective lens of a microscope is much smaller than that of a telescope and is spherical in shape, which produces a much shorter focal length.
Electron microscopes operate in a manner similar to optical microscopes, but they make use of lens that are designed to focus electrons rather than optical light. These microscopes are very powerful and are able to produce a level of magnification that is many orders of magnitude higher than that of an optical microscope. Both optical and electron microscopes are able to make use of different lenses to provide various levels of magnification.