Microscopy and types of microscope

The cells are very minute and complex organisations.The small dimensions and transparent nature of cell and its organelles pose problems to cell biologists trying to understand its organisation and functioning.Various instruments and techniques have been developed to study cell structure, molecular organization and function.

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The diameters of majority of cells range from 5-500 micrometers,but most are between 10-150 micrometer.

While viewing objects,human eyes have limited distinguishing or resolving power.The ability to reveal minute details is expressed in terms of limit of resolution.

Types of microscope

Compound light microscope

This microscope uses visible light for illuminating the object.It contains glass lenses that magnify the image of the object and focus the light on the retina of the observer’s eye.It has two lenses one at each end of a hollow tube.The lens closer to the object being viewed is called objective lens.The lens closer to the eye is called ocular lens or eyepiece.The object is illuminated by light beneath it.A third lens called condenser lens is located between the object and the light source and it serve to focus the light on the object.

Dark field microscope

This type of microscope is useful for viewing suspensions of bacteria.It has a special condenser that allows only rays of light scattered by structures within specimen.The result is an image that appears bright against background,with a high degree of contrast.The process is similar to seeing dust particles floating in a Sunbeam.

Phase contrast microscope

The phase contrast microscope has special fitments to the objective lens and such condenser,the effect of which is to exaggerate the structural differences between the cell components As a consequence,the structures within living,unstained cells become visible in high contrast and with good resolution.Phase contrast microscopy avoids the need to kill cells or to add dye to a specimen before it is observed microscopically.

Oil immersion microscopy

In oil immersion microscopy the light gathering properties of the objective lens are enhanced by placing oil in the space between the slide and objective lens.Normally the technique is used to view permanently mounted specimens.The oil immersion lens gives higher magnification than the normal high power objective lens.

Electron microscopy

The electron microscopy uses the much shorter wavelengths of electrons to achieve resolutions as low as 3 angstrom.Electromagnetic coils are used to control and focus a beam of electrons accelerated from a heated metal wire by high voltages,in the range of 20,000 to 100000 volts.The degree of which electrons are scattered is determined by the thickness and atomic density of the object.Hence the specimens used in electron microscopy must be extremely thin.Living cells which are wet cannot be viewed in electron microscope.

Scanning electron microscopy

This microscope has less resolution power than the TEM.However it is a very effective tool to study the surface topography of a specimen.The whole specimen is scanned by a beam of electrons.An image is created by the electrons reflected from the surface of the specimen.Scanning electron micrographs show depth of focus and a three dimensional image of the object.