MICROSCOPES

SIMPLE MICROSCOPE

A Simple microscope is commonly used in a laboratory. It is most usually used for dissection of small organisms or for getting the magnifying look at certain organs of organisms e.g. while studying mouthparts or insects. In a simple microscope single or group of convex lenses is used. A convex lens magnifies the object and helps to produce a magnified image. Magnification obtained by simple microscope can be calculated as

Where D = the least distance of distinct vision

            F = Focal Length of convex length

This microscope can provide magnification only up to 3 times. In cases where greater magnification is required, more than one lens should be used.

A Simple microscope consists of two parts viz.

A.    Optical Part

B.    Mechanical Part

A.    Optical Part: It consists of a single convex lens

B. Mechanical Part: As the name suggests it provides the mechanical support to the microscope. It holds the lens at the proper position. A knob is present which helps in attaining proper magnification. A mirror is used for light adjustment and clips for holding the slide.

Several lenses can be fused to form a convex lens. Each lens here is called as ‘element’. Each element produces some distortion, but together all the distortions are nullified and a distortion-free image is obtained. Magnification up to 20 times can be obtained by such lenses. An example of such a lens is an achromatic triplet or planner lens. It consists of a double concave lens of crown glass fitted between two double convex lenses of flint glass. 

COMPOUND MICROSCOPE

In a compound microscope, a combination of two lenses is used to give magnification. It can give a magnification of 500 to 1000 times depending upon the combination of lenses used. It is also known as ‘Light microscope’.

There are two types of compound microscopes usually seen: monocular and binocular. As the name suggests monocular has a single eye-piece and binocular has two eye-pieces. Initially, all compound microscopes used a mirror to reflect the natural or artificial light but now, microscopes with battery-operated LED lights or microscopes working on electricity are also available. Apart from the source of illumination (natural light, battery-operated LED or electrically operated bulb), rest all the components of these microscopes are the same.

The parts of a microscope can be studied under the following sections:

                        I.         Mechanical / Structural parts

                      II.         Illumination and related parts

                    III.         Optical parts

                    IV.         Movable parts for Adjustment

Mechanical / Structural Parts:

          These parts of the microscope provide the basic framework and mechanical support to lenses and to the material to be studied under a microscope. There are three mechanical parts

1.   Footrest: It forms the base of a microscope which bears the weight of the complete microscope. It is generally used to hold the microscope while transportation or handling.

2.     Tube: It contains the lenses. It can be straight and fixed or bend and movable. Eye-piece is present at the top end of the tube while the bottom end objective is present. In the case where the tube is bend, a prism is present to direct the light rays. In binocular microscopes, two eye-pieces are present.

3.     Stage: It is used to clamp the slide at a place for observing. There is a hole at the center of the stage so that light rays can pass through the slide and reach our eyes. The slide can be fixated by pair of clamps or can be moved by the mechanical stage control which helps in the movement of slide in up-down and left-right directions.
Illumination and Related Parts:

            As the complete functioning of the microscope is based on light rays, this is an important part of the microscope. It has 4 parts

1.   Light source: Depending upon the type of microscope, a light source can be natural light, artificial light, battery-operated LED light or electrically operated light bulb. Battery operated and electrical light are easier to use as their intensities can be adjusted as per need.

2.   Mirror: A mirror is present in microscopes where natural light is used. The mirror is used to reflect the light rays. The mirror has plane and concave surfaces. The Concave surface is used when a condenser is absent.

3.    Condenser: A condenser is used for focusing of light rays. It is present between the stage and mirror.
4.  Diaphragm: It is used to adjust the light intensity in microscopes where natural light is used. It is absent in microscopes with artificial light as the source of illumination. It is present below the condenser. If used properly it reduces the blur effect caused due to light reflection.

Optical Parts

Optical parts consist of the lenses used for obtaining magnification. There are two lenses involved. The
one at the top end of the tube is called eye-piece while the one at the bottom end of the tube is called
objective.
1.   Eye-piece: The magnification power of the lens is engraved on the outer surface of the lens. Eye-piece generally has 5X or 10X power i.e. can magnify 5 times or 10 times respectively.

2.   Objective: Similar to eye-piece, the magnifying power of the lens is engraved on the outer side of the lens. Mostly, there are three objectives with magnifying powers 10X, 40/45X and 100X. The 100X objective is always used with the oil (cedarwood oil) and hence is also commonly known as Oil immersion objective. The 100X objective is immersed in a drop of oil, placed over the slide. This is done to reduce the refraction caused due to high magnification. As the magnifying power of objective increases, the distance between the slide and the objective lens decreases.

Movable parts for Adjustment:

These parts are for adjustment to focus the material under study. There are 5 parts

1.   Coarse Adjustment Screw: It is the largest screw. It is used to achieve an appropriate focus.

2.  Fine Adjustment Screw: As the name suggests it is for finer adjustment to get a clear focus. The objective moves slowly with this screw.

3.   Condenser adjustment screw: It is used to adjust the illumination. Raising the condenser results in increased illumination while lowering the condenser causes a decrease in illumination.

4.    Iris diaphragm lever: It is a small lever used for opening and closing of the diaphragm for increasing and decreasing light intensity.

5.    Mechanical stage control: It is used to move slide horizontally and vertically.                                         

ELECTRON MICROSCOPE

            As the name suggests, an electron microscope uses a beam of electrons as the source of illumination. An electron microscope is used for obtaining greater magnification as compared to the light microscope. This is possible because the wavelength of an electron is much shorter than that of photons. Hence, an electron microscope can reveal smaller and detailed structures to a greater extent than a light microscope. An electron microscope has a magnifying capacity up to 10⁷ X and resolving power of ~50pm.

            The construction of an electron microscope is analogous to that of the light microscope. Following are the parts in an electron microscope.

1.  Electron gun: it is the source of illumination. It is made up of tungsten metal coil. At high temperatures, the tungsten metal coil produces a continuous beam of electrons.

2.     Metal column: It is the column where the complete assembly of the electron microscope is present.

3.   Electromagnetic coils (condenser lens): these are the metal coils which produce electromagnetic field when high voltage electric current is passed through them. The magnetic field produced by the electromagnetic coils helps in deflecting the beam of electrons to the desired extent. Thus is used to achieve the desired magnification.

4.     Fluorescent screen: It is the screen where the beam of electrons fall and is converted into an image.

5.    Transformer: A transformer is required in order to use high voltage electric currents in the electron microscope.

6.     Vacuum: The entire assembly of the electron microscope is present in a vacuum.

7.  Cooling system: a lot of heat is generated by the high voltage electric currents. Thus, the colling system helps in the reduction of the heat generated. The cooling system may use water or air for cooling.

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