Types of microscopes
Microscopes classification is based on the types of light source used and consists of
two main categories; optical microscopes that utilize visible light and microscopes
that utilize source other than visible light. This paper will only discuss microscopes
that utilize visible light. There are five different kinds of microscopes that utilize
visible light as their source of illumination.
The first is an Optical Microscope. This microscope acts as a two stage-magnifying
device. An objective lens, the lens nearest to the object being viewed, provides the
initial enlargement and an ocular lens, the lens nearest to the eye, is placed so as to
magnify the primary image a second time. Total magnification is obtained by
multiplying the magnifying power of the objective and ocular lenses. An additional
condensing lens is normally employed beneath the stage of the microscope to
concentrate the light from its source into a very bright beam illuminating the object,
thus providing sufficient light for inspection of the magnified image.
A Polarizing Microscope is used for natural objects such as
crystals & fibers that exhibit a special optical property
known as double refraction. Double refraction is caused by
asymmetric particles, too small to be resolved even by best
possible lenses. The polarizing microscope is a
conventional microscope in which a nickel prism is
interposed in the light path below the condenser, see
figure 7. This “Polarizer” converts all the light passing
through the instrument into plain polarized light. A
similar second prism termed “analyzer” is placed within
the barrel of the microscope above the objective lens.
When the analyzer is rotated until its axis is perpendicular to that of the polarizer,
no light can pass through the ocular lens, resulting in a dark field effect. The field
will remain black if a single refractive object is placed on the stage. A double
refraction object, however, will appear bright upon a dark background when
examined in this manner.
A Phase Contrast Microscope has solved the problem of lack of contrast in biological
work. Normally staining samples solves this problem but this creates numerous
limitations. Phase microscopy provides a method whereby contrast is created by
purely optical means. Refractive index is the measure of optical density of an object
or the speed with which it is traversed by the light wave. Air e.g. has a refractive
index of approximately 1.0, Water 1.3 and a glass about 1.5. In other words, light
traverses fastest in air, more slowly in water and slower still in glass. Light waves
traversing equal distance through air, water and glass will not emerge at
the same time; they will emerge out of the phase with each other.
The phase contrast apparatus consist of optical plates within the
condenser and objective lens that converts the phase differences
into amplitude differences, so that differences in refractive index
are rendered directly visible. Objects ordinarily transparent
become visible through contrast difference. Phase contrast
Microscope has application chiefly in the study of living cells, tissues
and of unstained, plastic embedded sections.
Interference Microscope:
Interference Microscope depends upon the ability of an object to retard light. The
interference microscope sends two separate beams of light through the specimens,
which are then combined in the image plane. After recombination, the differences in
retardation of light result in interference that can be used to measure the thickness
of the object under investigation.
Dark field Microscope utilizes a strong, oblique light that does not enter the
objective lens. A special dark field condenser, in which no light passes through the
center of the lens, is employed. Light thus reaches the object to be viewed at an
angle so oblique that none of it can enter the objective lens. The field is therefore
dark. However small particles present in the specimen will reflect some light into
the objective lens and will appear as glistening spots. Thus, it is possible to
visualize particles far below the limits of bright light resolution. The effect is
similar to phenomenon of dust particles seen in a beam of sunlight entering a
darkened room. It is useful in the examination of small transparent objects such as
chylomicron (particles of fat in the blood) that are invisible in the glare of bright
field examination.
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