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Optical microscope

2007 Schools Wikipedia Selection. Related subjects: Engineering; General
Physics

   The optical microscope is a type of microscope which uses visible light
   and a system of lenses to magnify images of small samples. Optical
   microscopes are the oldest, simplest and.

Optical configurations

   There are two basic configurations of optical microscope in use, the
   simple (one lens) and compound (many lenses).

Simple optical microscope

   A simple microscope is a microscope that uses only one lens for
   magnification, and is the original light microscope. Van Leeuwenhoek's
   microscopes consisted of a single, small, convex lens mounted on a
   plate with a mechanism to hold the material to be examined (the sample
   or specimen). Demonstrations by British microscopist Brian J. Ford have
   produced surprisingly detailed images from such basic instruments. The
   use of a single, convex lens to magnify objects for viewing is found
   today only in the magnifying glass, the hand-lens, and the loup.

Compound optical microscope

   The compound microscope uses a set of many lenses in order to maximize
   magnification. The diagram below shows a compound microscope. In its
   simplest form—as used by Robert Hooke, for example—the compound
   microscope would have a single glass lens of short focal length for the
   objective, and another single glass lens for the eyepiece or ocular.
   Modern microscopes of this kind are usually more complex, with multiple
   lens components in both objective and eyepiece assemblies. These
   multi-component lenses are designed to reduce aberrations, particularly
   chromatic aberration and spherical aberration. In modern microscopes
   the mirror is replaced by a lamp unit providing stable, controllable
   illumination.
   Basic microscope main elements: 1. ocular lens or eye-piece 2.
   objective turret, or nosepiece 3. objective lenses 4. coarse adjustment
   knob 5. fine adjustment knob 6. object holder or stage 7. mirror 8.
   diaphragm and condenser
   Enlarge
   Basic microscope main elements:
   1. ocular lens or eye-piece
   2. objective turret, or nosepiece
   3. objective lenses
   4. coarse adjustment knob
   5. fine adjustment knob
   6. object holder or stage
   7. mirror
   8. diaphragm and condenser

History of the microscope

   Compound microscope made by John Cuff in 1750
   Enlarge
   Compound microscope made by John Cuff in 1750

          See timeline of microscope technology.

   It is impossible to say who invented the compound microscope. Dutch
   spectacle-makers Hans Janssen and his son Zacharias Janssen are often
   said to have invented the first compound microscope in 1590, but this
   was a declaration by Zacharias Janssen himself halfway through the 17th
   century. The date is certainly not likely, as it has been shown that
   Zacharias Janssen actually was born around 1590. Another favorite for
   the title of 'inventor of the microscope' was Galileo Galilei. He
   developed an occhiolino or compound microscope with a convex and a
   concave lens in 1609. Galilei's microscope was celebrated in the ´Lynx
   academy´ founded by Federico Cesi in 1603. Francesco Stelluti's drawing
   of three bees were part of pope Urban VIII´s seal, and count as the
   first microscopic figure published (see Stephen Jay Gould, The Lying
   stones of Marrakech, 2000). Christiaan Huygens, another Dutchman,
   developed a simple 2-lens ocular system in the late 1600's that was
   achromatically corrected and therefore a huge step forward in
   microscope development. The Huygens ocular is still being produced to
   this day, but suffers from a small field size, and the eye relief is
   uncomfortably close compared to modern widefield oculars.

   Anton van Leeuwenhoek (1632-1723) is generally credited with bringing
   the microscope to the attention of biologists, even though simple
   magnifying lenses were already being produced in the 1500's, and the
   magnifying principle of water-filled glass bowls had been described by
   the Romans (Seneca). Van Leeuwenhoek's home-made microscopes were
   actually very small simple instruments with a single very strong lens.
   They were awkward in use but enabled van Leeuwenhoek to see highly
   detailed images, mainly because a single lens does not suffer the lens
   faults that are doubled or even multiplied when using several lenses in
   combination as in a compound microscope. It actually took about 150
   years of optical development before the compound microscope was able to
   provide the same quality image as van Leeuwenhoek's simple microscopes.
   So although he was certainly a great microscopist, van Leeuwenhoek is,
   contrary to widespread claims, certainly not the inventor of the
   microscope.

The components of the microscope

   All optical microscopes share the same basic components:
     * The eyepiece or ocular - a cylinder containing two or more lenses
       to bring the image to focus for the eye. The eyepiece is inserted
       into the top end of the body tube. Eyepieces are interchangeable
       and many different eyepieces can be inserted with different
       magnifications. Typical values for eyepieces include X5, X10 and
       X20. In some high performance microscopes, the optics of the
       objective and eypiece are matched to give the best possible optical
       performance. This occurs most commonly with apochromatic objectives
     * The objective lens - a cylinder containing one or more lenses to
       collect light from the sample. At the lower end of the microscope
       tube one or more objective lenses are screwed into a circular nose
       piece which may be rotated to select the required objective lens.
       Typical values of objectives are x5, x10, x20, x40, x80 and x100.
       Some high performance objectives may require matched eyepieces to
       deliver the best optical performance.
     * The stage - a platform below the objective which supports the
       specimen being viewed. In the centre of the stage is a circular
       hole through which light shines to illuminate the specimen. The
       stage usually has arms to hold slides (rectangular glass plates
       with typical dimensions of 25mm by 75mm, on which the specimen is
       mounted).
     * The illumination source - below the stage the light is provided and
       controlled in a variety of ways. At its simplest, daylight is
       directed via a mirror. Most microscopes, however, have their own
       controllable light source that is focused through an optical device
       called a condenser with diaphragms and filters available to manage
       the quality and intensity of the light.

   In the classic microscope the whole of the optical assembly is attached
   to a rigid arm which in turn is attached to a robust U shaped foot to
   provide the necessary rigidity. The arm is usually able to pivot on its
   joint with the foot to allow the viewing angle to be adjusted. Mounted
   on the arm controls for focusing are usually placed, typically a large
   knurled wheel to control coarse focusing together with a smaller
   knurled wheel to control fine focusing.

   More modern microscopes may have many more features, including
   transmission/reflection illumination, filters, apparatus for phase
   contrast microscopy and differential interference microscopy, digital
   cameras, etc.

Use of the microscope

   Compound optical microscopes are typically used to examine a smear, a
   squash preparation, or a thinly sectioned slice of some material. With
   a few exceptions, they utilize light passing through the sample from
   below and special techniques are usually necessary to illuminate the
   sample to increase the contrast in the image to useful levels (see
   contrast methods). However, at low power they may also be used to
   examine small living animals and plants and even at high power,
   microscopic living speciments such as protozoa, rotifers, and bacteria
   may be examined.

   A common use of non-transmitted lighting is to study the thin structure
   of metals (see metallography) and minerals, where the light is
   reflected from the examined surface.  The light is fed down through the
   objective using a semi-transparent mirror, and the reflected light
   observed as normal.

   Typically, on a standard compound optical microscope, there are three
   objective lenses: a scanning lens (5×), low power lens (10×) or
   sometimes medium power lens (20x), and high power lens (40×). Advanced
   microscopes often have a fourth objective lens, called an oil immersion
   lens. To use this lens, a drop of immersion oil is placed on top of the
   cover slip, and the lens is very carefully lowered until the front
   objective element is immersed in the oil film. Such immersion lenses
   are designed so that that the refractive index of the oil and of the
   cover slip are closely matched so that the light is transmitted from
   the specimen to the outer face of the objective lens with minimal
   refraction. An oil immersion lens usually has a power of 100×. Older
   microscopes were sometimes equipped with water immersion lenses which
   had a higher performance than non-immersion lenses but which were still
   significantly inferior in performance to oil immersion lenses.

   The actual power or magnification of an optical microscope is the
   product of the powers of the ocular ( eyepiece), usually about 10×, and
   the objective lens being used.

   Compound optical microscopes can produce a magnified image of a
   specimen up to 1000× and, at high magnifications, are used to study
   thin specimens as they have a very limited depth of field.

How a microscope works

   How a microscope works
   Enlarge
   How a microscope works

   The optical components of a modern microscope are very complex and for
   a microscope to work well, the whole optical path has to be very
   accurately set up and controlled. Despite this, the basic optical
   principles of a microscope are quite simple.

   The objective lens is, at its simplest, a very high powered magnifying
   glass i.e. a lens with a very short focal length. This is brought very
   close to the specimen being examined so that the light from the
   specimen comes to a focus about 160 mm inside the microscope tube. This
   creates an enlarged image of the subject. This image is inverted and
   can be seen by removing the eyepiece and placing a piece of tracing
   paper over the end of the tube. By careful focusing a rather dim image
   of the specimen, much enlarged can be seen. It is this real image that
   is viewed by the eyepiece lens that provides further enlargement.

   In most microscopes, the eyepiece is a compound lens, which is made of
   two lenses one near the front and one near the back of the eyepiece
   tube forming an air separated couplet. In many designs, the virtual
   image comes to a focus between the two lenses of the eyepiece, the
   first lens bringing the real image to a focus and the second lens
   enabling the eye to focus on the now virtual image.

   In all microscopes the image is viewed with the eyes focused at
   infinity. Headaches and tired eyes after using a microscope are usually
   signs that the eye is being forced to focus at a close distance rather
   than at infinity.

Stereomicroscope

   Stereo microscope
   Enlarge
   Stereo microscope
   Scientist using a stereo microscope outfitted with a digital imaging
   pick-up
   Enlarge
   Scientist using a stereo microscope outfitted with a digital imaging
   pick-up

   The stereo or dissecting microscope is designed differently from the
   diagrams above, and serves a different purpose. It uses two separate
   optical paths with two objectives and two eyepieces to provide slightly
   different viewing angles to the left and right eyes. In this way it
   produces a three-dimensional (3-D) visualization of the sample being
   examined.

   The stereo microscope is often used to study the surfaces of solid
   specimens or to carry out close work such as sorting, dissection,
   microsurgery, watch-making, small circuit board manufacture or
   inspection, and the like.

   Great working distance and depth of field here are important qualities
   for this type of microscope. Both qualities are inversely correlated
   with resolution: the higher the resolution (i.e., magnification), the
   smaller the depth of field and working distance. A stereo microscope
   has a useful magnification up to 100×. The resolution is maximally in
   the order of an average 10× objective in a compound microscope, and
   often much lower.

   The stereo-microscope should not be confused with ordinary compound
   microscopes equipped with a binocular eyepieces. In these microscopes
   both eyes can see the image but the binocular head provides greater
   viewing comfort and slightly better appearance of resolution. However
   the image in such microscopes remains monocular.

Special designs

   Other types of optical microscope include:
     * the inverted microscope for studying samples from below; useful for
       cell cultures in liquid;
     * the student microscope designed for low cost, durability, and ease
       of use;
     * the research microscope which is an expensive tool with many
       enhancements;
     * the petrographic microscope whose design usually includes a
       polarizing filter, rotating stage and gypsum plate to facilitate
       the study of minerals or other crystalline materials whose optical
       properties can vary with orientation.

   An old pocket microscope
   Enlarge
   An old pocket microscope
     * the polarizing microscope
     * the fluorescence microscope
     * the phase contrast microscope

Limitations of light microscopes

   Compound optical microscopes are limited in their ability to resolve
   fine details by the properties of light and the refractive materials
   used to manufacture lenses. A lens magnifies by bending light (see
   refraction). Optical microscopes are restricted in their ability to
   resolve features by a phenomenon called diffraction which, based on the
   numerical aperture (NA or A[N]) of the optical system and the
   wavelengths of light used (λ), sets a definite limit (d) to the optical
   resolution. Assuming that optical aberrations are negligible, the
   resolution (d) is given by:

          d = \frac { \lambda } { 2 A_N }

   Usually, a λ of 550 nm is assumed, corresponding to green light. With
   air as medium, the highest practical A[N] is 0.95, and with oil, up to
   1.5.

   Due to diffraction, even the best classic optical microscope is limited
   to a resolution of 0.2 micrometres.

   Optical microscopes have a focal point, either chosen or fixed, where
   the image is clear. This covers a two-dimensional area only. A single
   optical image cannot capture all the details of a three-dimensional
   shape in focus. Other types of microscopes are capable of imaging
   three-dimensional shapes.
   Retrieved from " http://en.wikipedia.org/wiki/Optical_microscope"
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   with only minor checks and changes (see www.wikipedia.org for details
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