VMC 211: General Veterinary Microbiology (1+1)

Microscope types

TYPES OF MICROSCOPE

Dark field microscope

• It is used for examining live organisms that are either invisible in ordinary light microscope or can not be stained by standard methods. In a dark field microscope instead of normal condenser, a dark field condenser is used, which is a light opaque disc. This disc blocks the light that enters the objective directly. Only light that is reflected off the specimen enters the objective lens. Because of this the background appears black. This technique is frequently used to see unstained microorganisms in liquid. Spirochetes are also examined by this method.

Phase contrast microscope

• Phase contrast microscopy is normally used to study the internal structures of living microorganisms. In this method the specimen need not be fixed and stained. The principle is based on slight variation in the refractive index. As light rays pass through the specimen, their velocity may be altered by differences in the thickness and physical properties of various parts of the specimen. Light rays passing through the specimen are diffracted differently and travel different paths to reach the eye. These phase differences are seen through the microscope as different degrees of brightness. Details of the specimen also become more sharply defined. The internal details appear as degrees of brightness against a dark background. A phase contrast microscope uses a special condenser with annular ring shaped diaphragm which allows a ring of light to pass through the condenser, thus focusing light on the specimen and on a ring shaped diffractive plate in the objective lens. The diffracted and undiffracted rays are then brought back in to phase with each other to produce image.

Fluorescence microscope

• Fluorescence is the emission of a higher wave length by some substances after absorbing light of a particular wave length. This phenomenon is used in fluorescence microscope. In this microscope instead of normal light blue light is used. The specimen is stained by fluorescent dyes like rhodamine or fluorescein isothiocyanate (FITC). These dyes fluoresce when exposed to blue light. Rhodamine produces red fluorescence and FITC produces green fluorescence. This microscopy is normally used in immunology to study antigen antibody reaction.

Electron microscope

• There are two types of Electron microscopes namely, Transmission electron microscope (TEM) and Scanning electron microscope (SEM).
• In transmission electron microscope electron beam is used to magnify the specimen instead of light. The wave length of the electrons used is very low (0.005 nm). Hence, the resolving power of electron microscope is more than 100 times of light microscope. Magnification up to 4,00,000 X can be produced.
• In scanning electron microscope a narrow beam of electrons moves over the surface of the specimen. This leads to release secondary electrons and other types of radiation from the specimen surface. The intensity secondary electrons vary based on the shape and the chemical composition of the specimen. A detector produces signals by collecting the secondary electrons. These signals are used to produce an image on cathode ray tube. The resolving power of scanning electron microscope is lesser than that of transmission electron microscope.