Microscopy, optical and electron microscope with ray diagram

The closest distance of separation (i.e. resolution) visible to the human naked eyes is 0.1 mm. Inside details of solids, crystals, unit cells, atoms, electrons, and imperfection demand for visibility of much smaller dimensions. The smaller dimensions may be as small as 1 Angstrom(10^-10m) or less. These situations need a magnified vision. This is possible through a microscope. Microscopes of various magnification ratio are used according to the need. Their magnification generally varies between 5 to 1000000. Microscopes are broadly classified into following main categories.

1. Optical microscope

2. Electron microscope

3. Field ion microscope

4. Scanning tunneling microscope

Here we have listed some magnification and applications of different types of microscopes

Type	Range of magnification	Used in study of
Optical microscope	10 to 2000 times linear	Microstructure
Electron microstructure	100000 times linear	Finer particles, dislocations
Field ion microscope	upto 1000000 times	Imperfections
Scanning tunneling microscope	more than 1000000 times	Atomic image microscope

Microscopic principle

The microscopic examination is based on optical principle. In it the rays from light source is passed on to a glass reflector through a diffusing disc and an Iris diaphragm as shown in the picture. The diffusing disc helps in diffusing the light, the Iris diaphragm controls the width of light beam, and the reflector kept at 45^o partially reflects the light rays onto the sample (or object).

An optical microscope illustrating the principle of microscopy

After illuminating the polished sample; the rays return by reflection, pass through the objective and glass reflector, and then form an image. This image can be seen through eye-piece to get the view of sample surface.

 The eye piece is carried by a ‘draw tube’ at its top end. The draw tube can slide within the ‘body tube’ of microscope through a rack and pinion mechanism, on rotating the coarse and fine adjustment knobs. By doing so the distance between objective and the eye-piece can be varied for focusing the object. The coarse adjustment is done for initial focusing  and the fine adjustment for final focusing.

Ray diagram and principle of Magnification

In a metallurgical microscope the objective is provided for resolving the sample structure where as eye-piece enlarges the image formed by objective.  Picture shown below depicts a schematic arrangement of an objective and eye-piece to explain the formation of magnified image in a metallurgical microscope. The travel path of light rays is also shown in this diagram which is self-explanatory. Its optical combination may bring-out a magnification of 50* to 1500* (* denotes linear magnification).

Magnifying power of microscope: Magnifying power of a microscope is its ability to enhance the size of real object many times. In an optical microscope, the objective and the eye-piece both magnify the real object. Therefore, magnifying power of objective is different from the magnifying power of eye-piece. Objectives are generally available in magnifying powers of 10*, 40* and 1000*.

Where * denotes a linear magnification. Eye piece s are generally available in magnifying powers of 5*, 6*, 7.5*, 10*, 15*, 20* and 25*.

In the picture shown below, AB is an object whose magnified image is CD due to objective lens. The image of CD is further magnified to EF by eye piece. The total magnification M_t is obtained by combining magnifications of the objective and the eye piece. This is expressed better with the help of picture shown below.

Ray diagram for image formation in metallurgical (optical) microscope

                                                        M_t=M_oM_eD*

Where M_o=CD/AB and M_e=EF/CD are magnifications of objective and eye piece respectively, and D is the projection distance. Projection distance is measured between the eye lens of eye piece and the screen on which image is projected. Generally the objectives are designed for use at a definite tube length l. Hence equation 1 modifies to M_t=(M_oM_e)/l*

(Here * denotes the magnification)

The value of l=250mm is very common.

Working principle of Electron Microscope

This microscope is very useful tool in crystallographic research. It can produce photographic images and diffraction patterns. It employs much shorter wavelength (about 1/20 of X-rays). Therefore, it results in better resolution of photographic image than those through an optical microscope. Schematic layout of an electron microscope is shown in picture below. It has much higher power of resolution (PR). The object is illuminated by a beam of electrons. It employs magnetic focusing.

Formation of magnified image

In this operation, the tungsten filament T is thermionically heated. Due to this the electrons are emitted which are collimated by metallic grid M. The collimated electron beam is accelerated by anode A to a potential about 5000 volts. Accelerated beam is then focused on the object by the magnetic condenser coil. The object is placed on a cellulose film held in a holder. The incident electron beam on the object scans it, and then the objective coil produces a magnified image I₁ of the object. This image I₁ acts as an object for projector coil which magnifies it to image I₂. Final image I₂ may be seen on a fluorescent screen or on a photographic plate.

Layout of electron microscope

Whole system is placed in a metal casing which is evacuated to produce vacuum. Arrangements of keeping and moving the object and its adjustment are incorporated in the microscope. Electron microscope may be used in the fields of medicine and biology to study bacteria and virus, in colloidal solutions to examine minute particles, in textile industry to study structure of fibres, and in industries like paper, paints, plastics, lubricants and metals.