Different types of microscopes are used by the microbiologists for specific purposes. Microscopes are of the following types
A. Light or Optical Microscopes:In light or optical microscopes, ‘light waves’ are used to produce the enlarged images of very small objects and magnification is obtained by a system of ‘optical lenses’. Ordinarily, microbes do not absorb much light, but staining them with dyes greatly increases their light absorbing ability, resulting in greater contrast and color differentiation.
Light microscopes are of four types as described below:
(1) Bright-field Microscope:In a bright-field microscope, the microscopic field (the circular area visible under microscope) is brightly illuminated and the microbes (or biological specimen) appear darker, as they absorb some of the light passing through them.It is of two types as follows:
(a) Simple Microscope:A simple microscope is used to obtain small magnifications. A single biconvex lens magnifies the size of the object to get an enlarged virtual image.
(b) Compound Microscope:The most commonly used microscope for general purposes is the standard compound microscope. It magnifies the size of the object by a complex system of lens arrangement. It has a series of two lenses; the objective lens and the ocular lens, to magnify the size of the object.
2. Dark-field Microscope:In a dark-field microscope, the object is brilliantly illuminated against a dark background.
3. Fluorescence Microscope:It is used to view fluorescent microbes and microbes stained with fluorescent dyes for specific purposes.
4. Phase-contrast Microscope:With a phase-contrast microscope, the differences among various cells with different refractive indices or thickness can be seen in unstained condition. Unstained structures within cells, not discernible by most other microscopes can also be observed, due to the slight differences in their refractive indices or thickness. It is a compound microscope fitted with a phase-contrast condenser and a phase-contrast objective.
B. Electron Microscopes:
In electron microscopes, ‘electron beams’ are used to produce the image of the object and magnification is obtained by a system of ‘electromagnetic fields’, unlike in light microscopes, in which Tight waves’ are used to produce the image of the object and magnification is obtained by a system of ‘optical lenses’.The resolving power of electron microscopes is 200 times greater than that of light microscopes. They can produce useful magnifications up to X 400,000, as compared to X 2000 in light microscopes. Thus, the useful magnification is 200 times greater in electron microscopes than in light microscopes.
Electron microscopes are of three types as described below:
1. Transmission Electron Microscope (TEM):In this microscope, electron beam is transmitted through an ultra-thin section of the object and the image is magnified by the electromagnetic fields. It is used to observe finer details of internal structures of microscopic objects like bacteria and other cells.
2. Scanning Electron Microscope (SEM):This microscope uses an electron beam to scan the surface of the object, thereby inducing it to release a shower of electrons, which are collected by a detector to generate the image. It is used to observe the surface structure of microscopic objects.
3. Scanning and Transmission Electron Microscope (STEM):It has both transmission and scanning electron microscope functions.