Microscope and its components

                            MICROSCOPY

The field of designing and using microscopes to view objects that cannot be seen with unaided eye is known as microscopy.

 Hans Zacharius Janssen a Dutch spectacle maker around 1590 used two magnifying lenses to magnify images without any provision for the focusing.

Compound microscopes were later improved upon and used by Galileo (1609) with the provision for the focusing.

Robert hook (1665) invented the compound microscope with improved quality and he wrote the book ‘Micrographia’ and he took the observation of white mould.

However the word of microorganism was opened to human civilization by the first time by Dutch merchant Antony van leeuwenhoek(1632-1723).

He is generally given credit as being the first to see and describe bacteria.

Faber in (1625) used the term microscope.

                              Microscope

It is an optical instrument consisting of a lens or a combination lenses for making enlarged or magnified image of minute objects. 

The term microscope is derived from two Greek words Micro- Small Scope- to see or to look.

Types

 A wide variety of microscopes are available based on the principle involved in the magnification.

 All kinds of microscope are classified under the following two main types

1)Light microscope

2)Electron microscope

Light microscope: in this type light rays are used as source of illumination and magnification of the object and can be achieved with the optical lenses hence it is also called optical microscope.

✓ Light microscope include 

✓ Bright field microscope

✓ Dark field microscope

✓ Phase contrast microscope

✓ Fluorescence microscope

✓ Stereo microscope

Bright field compound microscope: in this type of microscopy microscopic field is bright in which object or specimen will remain dark.

Simple microscope: consist of only one lens or magnifying glasses held in a frame.

Principle of simple microscope: it consists of a single convex lens which magnifies the object.

Compound microscope: Consists of more than one lens or magnifying glasses

Principle of compound microscope: when the light rays passes through the lenses they refract (converge) and produce magnified image of the specimen when passes through it differ from a simple microscope in having two set of lenses one is known as objective and other as eye piece.

Construction: It consists of two parts namely, optical part and Mechanical part

Optical parts: includes condenser lens system objective lens system, eye pieces or ocular lens system. 

Mechanical parts: includes body tube, coarse adjustment, fine adjustments, arms, nose pieces, stage, stage clips, Diaphragm, inclination joint, mirror, and base.

Eye pieces: it consist of two convex lens of which lower bigger one is called field lens and upper smaller one is called eye lens. These lenses magnify the primary image of the object and produce secondary image which is picked up by the viewer.

Body tube: it is a cylindrical tube holds eye pieces at one end and objective at the other end at the proper working distance. It is metallic part and draw tube is provided with a millimetre scale, which shows the total length of the working tube. 

Stage: It is a platform which accommodates the microscopic glass slide, on which the object to be examined is mounted. It has an aperture in the centre to permit light. The stage may be fitted with two metal clips, which are of mechanical type where by the slides can be moved by rack and pinion adjustment.

Coarse adjustment: it is a metallic part of the microscope which is a big circular screw it moves the nose pieces (body tube) up and down in order to rough focusing of the specimen or for a approximate focusing.

Fine adjustment: It is a metallic part of the microscope (small screw) which moves the nose pieces very slowly for definite sharp focusing of the specimen. 

Nose-piece: It bears different objective lenses and can be rotated to change from one objective to another according to the requirement.

Arm: it holds body tube, coarse adjustment and fine adjustment. It supports the upper half of the microscope.

Base: This is a shoe shaped metallic part and supports the entire microscope.

Irish diaphragm: it controls the amount of light striking the object. It can be opened and closed with the lever.

Eye-piece: It is an optical part of the microscope and also called an ocular. It has two lenses.

1. Field lens (upper small lens)

2. Eye lens (lower larger lens)

Objective: It is an optical part of the microscope which is nearest to the stage. Microscope have three objectives which are fixed in a revolving nose pieces. 

Low power objective or 10X: It is an objective where the working distance is more. If the ocular is 10X, then the magnification is 100 times.

High power objective (10X or 45X): here the working distance is less as compare to low power. If the ocular is 10X then the magnification is 400 or 450 times. 

Oil immersion objective (100X)

 this requires the use of a drop of either cedar wood oil or liquid paraffin between the lens and the object. The oil serve to prevent loss of light rays due to refraction. It has a refractive index (RI) of 1.51. Here the working distance is very small.

Condenser  

It is an optical part which is made of lenses. It condenses the light rays here by preventing the escape of light rays. It also controls light intensity. 

Mirror: it has two reflecting surfaces. a. Plane, b. Concave.

The plane mirror reflects the light rays parallel to one another and the light source may be artificial light or tube lights. 

While concave mirror concentrate the light and light source may be the visible natural diffused light rays.

Inclination: it permits the tilting of the upper parts of the microscope.

Definitions: 

Resolving power: Ability of the lenses to distinguish two close dots as two separate dots is known as Resolving power. OR

The capacity of the microscope to distinguish between two paints lying very close to each other.

Resolving power of the objective lens of the microscope can be determined by using the formula 

Resolving Power (RP) = Wave length ( λ )

                                       2 x Numerical aperture

Numerical aperture: ( NA) It is the function of the effective diameter of an objective in relation to its focal length and refractive index of the medium between the specimen and the objective. 

NA = n sinΘ

Where n = Refractive index of the medium 

SinΘ : half the angle of cone of light that enters into the objective lens.

Maximum angle of cone of light that enters into the objective lens will be 900 and half the value of this is 450 and this will be consider as 

Then NA of the lens will be 1xn. The value of NA is depending on the nature of media present between the specimen and the objective.

Working distance: it is the distance between the specimen and the objective lens 

Electron Microscopy

M Knoll & E Ruska (1931) first invented electron microscope. In this type, a beam of electron are used in place of light rays as a source of illumination and magnification of the object 

Types: 

Transmission Electron Microscope (TEM): In this type of microscopy, electrons transmit through the specimen to produce the magnified image. Hence the name is TEM. It is used to study ultra structure of cell and its components.

Scanning Electron Microscope (SEM): In this type of microscopy, electrons scan the surface of the specimen and produce three dimensional images of the specimen, Hence the name is SEM. It is used to the surface structure of the specimen in living state without staining.