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# Microscope

 Common optical microscope and its principal parts
A microscope is an instrument for viewing objects that are too small to be seen by the naked eye.

The most common type of microscope, and the first to be invented, is the optical microscope. This is an optical instrument containing one or more lenses that produce an enlarged image of the object, and works by refraction (the bending of light). The science of investigating small objects using this instrument is called microscopy.

Optical microscopes are restricted in their ability to resolve features by a phenomenon called diffraction which, based on the numerical aperture (NA or $A_N$) of the optical system and the wavelengths of light used ($\lambda$), sets a definite limit ($d$) to the optical resolution[?]. Assuming that optical aberrations are negligible, the resolution is:

$d = \frac { \lambda } { A_N }$

Usually, a $\lambda$ of 550 nm is assumed, corresponding to green light. With air as medium, the highest practical $A_N$ is 0.95, and with oil, up to 1.5.