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The Principles And Limitations Of Transmission And Scanning Electron Microscopes

1149 words - 5 pages

The Principles and Limitations of Transmission and Scanning Electron Microscopes


Microscopy has a major role in cytology.From the very beginning
researchers have tried to develop ways of looking directly at living
cells.This examination has revealed much about the morphology of cells
and tissues.In recent years,development in microscopes,dyes,staining
and preparatory techniques have helped reveal even more about the
structure and function of cells.Microscopes have a certain
magnification and resolving power.In any microscope the the resolving
power is more important than the magnification.The resolving power of
a microscope is the least distance between two objects where the
microscope can still distinguish the objects as being separate.It is a
measure of detail that can be seen.A microscope with a high resolving
power enables us to view images with a high resolution.With a low
resolution they would be viewed as one object.Microscopes with a high
magnification are only able to increase the size of the object that is
being viewed.The resolution will be the same.(ie the object will still
lack clarity and appear fuzzy).The two types of microscopes are
electron and light microscopes.

Principles and Limitations of light microscopy

Light microscopes function by focussing a beam of light on the object
to be examined.The beam passes through a series of lenses after
passing through the specimen.The condenser lens focuses the light onto
the specimen for maximum illumination.The lower objective lens
magnifies the object ,and the image from this lens is further enlarged
when viewed through the upper eyepiece lens.The beam from the light
source directly enters the observer's eye.It has a few
limitations.Material to be examined by light microscopy must be
sufficiently transparent and thin enough for light to pass through
.They are cheap,portable and easy to handle.However they cannot
resolve anything that is less than 0.2 micro metre apart This limit is
due to the wavelength of light.For this reason they can't examine
minute organisms like viruses nor can they readily allow scientists to
examine individual tiny parts of cells in detail.It has low
resolution(200nm) and low magnification.(X1500).Howver light
microscopes only allow us to determine the shape of whole cells or
large organelles.It does not let us see smaller organelles.Those which
are visible lack clarity (appear fuzzy).Hence the magnification is
less important than the resolution.The low resolution of light
microscopes is due to the low wavelength of light (500nm)

Principles and Limitations of electron microscopy

The other type of microscope is the electron microscope (which
consists of transmission electron microscope and scanning electron
microscope).It works by focussing a beam of electrons at the

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