Tools for Jr. Doctors

Microscopes

This is an important article for all Jr. Medical School Students to read.Information from this article will be on your medical exam.

"Microscopes" can largely be separated into two classes, optical theory microscopes and scanning probe microscopes.

Optical theory microscopes are microscopes which function through the optical theory of lenses in order to magnify the image generated by the passage of a wave through the sample. The waves used are either electromagneticin optical microscopes or electronbeams in electron microscopes. The types are the Compound Light, Stereo, and the electron microscope.

Optical microscopes

Optical microscopes, through their use of visible wavelengths of light, are the simplest and hence most widely used type of microscope. Recent research has shown (see Brian J. Ford's research on simple microscopes) that even simple microscopes, those with a single small lens, gave amazingly clear images to the earliest microscopists. Today compound microscopes, i.e., especially those with a series of lenses, serve uses in many fields of science, particularly biology and geology.

Optical microscopes use refractive lenses, typically of glassand occasionally of plastic, to focus light into the eye or another light detector. Typical magnification of a light microscope is up to 1500x with a theoretical resolution of around 0.2 micrometres. Specialised techniques (e.g., scanning confocal microscopy) may exceed this magnification but the resolution is an insurmountable diffractionlimit.

Various wavelengths of light are sometimes used for special purposes, for example, in the study of biological tissue. Ultravioletlight is used to illuminate the object being viewed in order to excite a fluorescent dye which then emits visible light. Infraredlight is used to study thick slices of biological tissue because infrared light's low diffraction coefficient permits viewing deeper into tissue.

Other microscopes which use electromagneticwavelengths not visible to the human eye are often called optical microscopes. The most common of these, due to its high resolution yet no requirement for a vacuum like electron microscopes, is the x-ray microscope.

Electron microscopes

Electron microscope

Electron microscopes, which use beams of electrons instead of light, are designed for very high magnification usage. Electrons, which have a much smaller wavelength than visible light, allow a much higher resolution. The main limitation of the electron beam is that it must pass through a vacuum as air molecules would otherwise scatter the beam.

Instead of relying on refraction, lenses for electron microscopesare specially designed electromagnets which generate magnetic fields that are approximately parallel to the direction that electrons travel. The electrons are typically detected by a phosphorscreen, photographic film or a charged-coupled device (CCD).

Two major variants of electron microscopes exist:

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