A microscope is an optical device that uses magnification to allow the viewer to see objects otherwise invisible to the naked eye. A microscope uses three lenses to accomplish this task, an eyepiece lens, a condenser lens, and an objective lens. Depending on the type of microscope -- that is, compound vs. electron -- these lenses vary in terms of material, composition and power of magnification.
The eyepiece lens is the lens at the top of the microscope used for viewing. This lens usually has a power of 10x magnification but 5X, 15X and 20X types are also available. The strength of magnification depends on the focal length of the eyepiece; that is, the longer the eyepiece, the greater the magnification. Eyepiece lenses also vary in length. For instance, a widefield lens has a large diameter to allow a wider area of the field of view.
The condenser lens focuses the microscope's light source on the specimen. They are particularly useful for magnifications at 400x and above, in which the object under examination becomes fuzzy unless the light source is focused. Condenser lenses come in several varieties. An Abbe Condenser has an adjustable iris type aperture that controls the diameter of the light beam. An aplanatic condenser lens sharpens the view of the specimen by correcting spherical aberrations due to refraction of light rays. An acromatic condenser is used for color and image correction of the specimen in microscopes with very high magnification power. Condenser lenses for high-magnification microscopes have a numerical aperture, or NA, rating that expresses its ability to resolve fine detail in an object being observed.
The objective lens is the one closest to the specimen. Many microscopes have three or four of these lenses in varying strengths of magnification. A compound microscope usually has them in powers of 4x, 10x, 40x and 1000x. Objective lenses are built according to common specifications and may be interchangeable between microscopes. The more powerful objective lenses automatically retract in case they are hit by a slide to protect both the lens and the specimen.
Lenses and Microscope Types
The type of lens used in a microscope depends upon the type of microscope itself. For instance, compound microscopes, the common type you often encounter in a biology class, and dissection microscopes, which show a 3D image of the specimen, both use glass lenses. A confocal microscope uses laser light to scan a digital image of the specimen through glass lenses with dichromatic mirrors. Electron microscopes use electrostatic and electromagnetic lenses to magnify an image by bouncing electrons off the specimen.