THE MICROSCOPE.

THE MICROSCOPE

Much of the living world is too small for human eyes to see. Our eyes can only see objects that are larger than 0.1mm. Objects with sizes smaller than 0.1mm can be viewed by using microscopes.

What is a microscope?
A microscope is an instrument used for viewing objects which are too small to be seen by our naked eyes. It ranks as one of the most important tools of science.

• Physicians and biologists, for example, use microscopes to examine bacteria and blood cells.
• Material scientists and engineers use microscopes to study the crystal structures within metals and alloys (metal mixtures) and to examine computer chips and other tiny electronic devices.

There are two types of microscopes
a. Compound or light microscope
b. Electron microscope 

Optical or light microscope
An optical microscope has one or more lenses that refract (bend) the light rays that shine through or are reflected by the specimen being observed. The refracted light rays make the specimen appear much larger than it is. 

Magnifying glass is the simplest optical microscope, has only one lens. The best magnifying glasses can magnify an object by 10 to 20 times. 

The compound or light microscope
The compound or light microscope uses two or more sets of lenses to provide higher magnifications. Each set of lenses functions as a unit and is referred to as lens system. In microscopes with only one objective, the lens system and ocular are mounted at opposite ends of a tube. 

In microscopes with two or more objectives the objectives are mounted in a rotating nosepiece connecting to the end of the tube opposite the ocular. The person operating the microscope rotates the nose piece to align one of the objectives with the opening in the end of the tube. 

 

The workings of electron and compound microscope

• Electron microscope – Uses electrons to illuminate the specimen and can reveal much more structures than light microscope can do.

• Light microscope – Uses light to illuminate the specimen

LIGHT MICROSCOPE.

Parts of the light microscope and their functions 

Functions of parts of the light microscope:

1. Eyepiece – Magnify objects under observation since it consists of magnifying lenses. 

2. Body tube – Hollow tube attached to the arm. Its function is to hold eyepiece lens and revolving nosepiece. 

3. Revolving nosepiece – Holds objective lenses in place. Position of the objective lenses can be changed by manipulating the revolving nosepiece. 

4. Coarse adjustment knob – It lowers and raises the body tube so that a clear image is obtained. 

5. Fine adjustment knob – Raises and lowers the body tube to obtain a fine focus. 

6. Objective lens – Brings image into focus and magnifies it. 

7. Stage – This is a place where specimen to be observed is placed.

 
8. Clips – Hold the slide or specimen in position.

 
9. Mirror – Reflects and directs light to the object under observation. 

10. Diaphragm – Is an aperture that regulates the amount of light passing through the condenser to illuminate the specimen.

 
11. Condenser – Concentrates light reflected by the mirror.

12. Base or stand – Supports the microscope steadily.

 
13. Arm or limb – Supports the body tube and stage. It is used to hold the microscope.

 
14. Hinge screw – Raises and lowers the stage. 

Magnification

Magnification power is symbolized by a number and abbreviation X. For example a 10X magnifying glass magnifies an object by 10 times. An object is magnified by multiplying the eyepiece lens magnification and objective lens magnification.

Example:
Magnification = eyepiece lens x objective lens magnification = 10 × 20= X200

A table of magnification

Eye piece lens magnification	Objective lens magnification	Total magnification
5	20	X100
10	20	X200
15	10	X150
10	25	X250
20	20	X400

How to use a microscope

1. Turn on your microscope light 

2. Turn the nose piece so that the small (low power) objective lens clicks into place. Always start with low power lens in place. 

3. Place the prepared slide on the center of the stage under the clips so that the object is in the center of the opening. Make sure the cover slip is on top 

4. With your eye at stage level, use the coarse adjustment to bring the object and the low power objective lens as near to each other as possible. The objective lens should not touch the cover slip 

5. Now with your eye to the eyepiece, slowly move the coarse adjustment to increase the distance between the object and the lens. Continue this until the image is focused. 

6. Adjust the diaphragm so that the object can be seen as clearly as possible 

7. To observe the object under medium and high powers, rotate the revolving nosepiece to bring the next highest objective lens into position. Make sure you hear the 'click' to ensure that the objective lens is in place. Then, focus using the fine adjustment only. 

Ways of handling and carrying a light microscope

a. Use both hands to carry the microscope. One hand should hold the base and the other hand should hold the arm. 

b. Always place the microscope on the desk or table carefully and gently and never place it at the edge of the bench. 

c. Keep the microscope in an upright position when using liquids or when not in use. 

d. Keep the stage clean and dry. If any liquids are spilled on the microscope, wipe them up immediately with a piece of tissue. 

e. Focus with the low-power objective lens first. 

f. Focus by moving the lens away from the slide, that is, by increasing the working distance.

g. Consult your teacher if the lenses are dirty.(viii) Consult your teacher if the adjustments do not work freely. 

h. When your work is completed, move the low power objective lens into place and remove your slide. 

i. Keep your microscope covered when it is not in use and keep your work area clean and tidy.

ELECTRON MICROSCOPE

This type of microscope uses a beam of electrons rather than a beam of light to produce magnified images. Electron wavelengths are much shorter than those of visible light. As a result electron microscopes can resolve much finer detail than light microscope can do.


Types of electron microscopes

a. Transmission electron microscope (TEM) This type of a microscope passes a broad beam of electrons through a specimen slice a few hundred angstroms thick.

b. Scanning electron microscope (SEM) This microscope scans a focused beam across the surface of the specimen. 

Other kinds of microscopes

Scanning probe microscope The microscope scans a specimen with a sharp point called a probe.

The ion microscope (field -ion microscope) It is used to examine metals. It creates an image of the crystal structure of the tip of an extremely sharp metal needle. 

An electric field applied to the tip repels charged helium, neon or argon atoms which spread out and strike a special screen. 

The screen glows where the atoms strike it, forming an image of the arrangement of atoms in the metal.