1. Introduction to Microscopes
Since microorganisms are invisible to the unaided eye, the essential tool in microbiology is the microscope. One of the first to use a microscope to observe microorganisms was Robert Hooke, the English biologist who observed algae and fungi in the 1660s. In the 1670s, Anton van Leeuwenhoek, a Dutch merchant, constructed a number of simple microscopes and observed details of numerous forms of protozoa, fungi, and bacteria. During the 1700s, microscopes were used to further elaborate on the microbial world, and by the late 1800s, the sophisticated light microscopes had been developed. The electron microscope was developed in the 1940s, thus making the viruses and the smallest bacteria (for example, rickettsiae and chlamydiae) visible.

2. The smallest objects that the unaided human eye can see are about 0
The smallest objects that the unaided human eye can see are about 0.1 mm (=100 mm) long. That means that under the right conditions, you might be able to see an ameoba proteus, a human egg, and a paramecium without using magnification. A magnifying glass can help you to see them more clearly, but they will still look tiny.
Smaller cells are easily visible under a light microscope. It's even possible to make out structures within the cell, such as the nucleus, mitochondria and chloroplasts. Light microscopes use a system of lenses to magnify an image. The power of a light microscope is limited by the wavelength of visible light, which is about 500 nm. Typical magnification of a light microscope, assuming visible range light, is up to 1500x with a theoretical resolution limit of around 0.2 mm (= 200 nm). Therefore , most light microscopes can resolve bacteria but not viruses.
To see anything smaller than 200 nm, you will need an electron microscope. Electron microscopes shoot a high-voltage beam of electrons onto or through an object, which deflects and absorbs some of the electrons. Resolution is still limited by the wavelength of the electron beam, but this wavelength is much smaller than that of visible light. Electron microscopes, due to their shorter wavelengths (100,000 times shorter than light’s) allow greater magnification (10,000,000x) and higher resolution (50 picometers). The most powerful electron microscopes can resolve molecules and even individual atoms.

3. Different Types of Microscopes
A microscope is an instrument used for viewing small objects that can't be seen with the naked eye. Microscope comes from the Greek word micros meaning "small" and skopeîn meaning "to look at".
In general, there are three classes of microscopes, light or optical microscope, electron microscope, and scanning probe microscope.
From the very first microscope to the microscope available today, there is a great difference in the technology. Today you can find many different types of microscope which are capable of magnifying the object to a great extent. Following are the various types of microscope available which vary in magnification, resolution, illumination method, type of object, formation of image, depth of the field, etc.
Among these, optical microscope is the most commonly used type. It uses visible light and a coordination of lenses to magnify images. Simple and compound microscopes are light microscopes. While a simple model involves usage of only one lens, a compound microscope uses more than one lens.

4. Types of microscope
Compound Microscope Compound microscope is commonly used in the schools and colleges.
It comes under the category of types of microscopes used in biology.
It has two lenses namely the objective lens and the ocular lens.
It provides a magnification of 1500X
Eyepiece lens is of 10X or 15X power.
It is used to observe bacterial, protozoa, various cells, etc.
Light Microscope Light microscope is also called optical microscope.
Eyepiece lens is of 10X or 16X power.
It provides a magnification of 1500X.
Light microscope are the types of microscopes used in anatomy and physiology of minute creatures.
Dissecting Microscope It is also called stereo microscope.
Its magnification power is less than the other microscope.
It gives a 3 dimensional picture.
Due to the low magnifying power they are used to observe
little bigger objects than other microscopes.
They are used in the surgeries, dissection, forensic science etc.

5. Digital Microscope Digital microscope has a 2 mega pixel CCD camera which is attached to an LCD mionitor.
It has optical lens and also has CCD/CMOS sensors.
It provides a magnification of 1000X.
It is used to take high resolution pictures of the object.
Buying a digital microscope may cost you a lot but it will be worth it.
Electron Microscope Electron microscope has a high resolution as compared to the other microscopes.
It provides a beam of electrons which strikes the object and illuminates it.
It is one of the best type of microscopes used to study cells.
It is of two types: Scanning Electron Microscope and the Transmission Electron Microscope.
It works in vacuum which reduces the chances of electrons to strike other air molecules.
Transmission Electron Microscope It provides very high magnification.
It uses electron beam to magnify the specimen.
It provides a 2 dimensional image.
The specimen in sliced ultra thin and then it is kept on a wire grid. The electrons strike the object which makes it visible.
The object is seen dark over the light background.

6. Scanning Electron Microscope It is also a type of electron microscope.
It has a low magnifying power than the transmission electron microscope.
You can get a 3 dimensional image with this microscope.
To get a proper view specimen is stained with gold and palladium.
The picture found is in black and white.
Dark-field Microscope Special condenser lens is used to magnify object.
It is known as dark-field microscope because the image is formed on a dark background.
The light falls on the specimen and scatters further forming the image.
It is used to observe live specimens for example spirochetes which causes syphilis.
Phase Contrast Microscope This microscope also works with the help of special condenser and phase plate.
The light strikes the object at different speeds.
You can see the unstained and live microorganisms in this microscope.
You can also observe various parts of the cell like
the mitochondria, lysosomes, golgi bodies, nucleus etc.
Fluorescent Microscope It works with the help of ultraviolet light.
The ultraviolet light strikes the specimen and excites electrons of the object which can be seen in different colors.
Fluorescent dyes are used to highlight the object.
The ultraviolet light increases the resolution.
It is very helpful in identifying the microorganism

7. Dark-field Microscope

9. The invention of simple microscope is credited to Van Leeuwenhoek, the Father of Microbiology, who was the first to observe single-celled microorganisms.
Later, Galileo Galilei, an Italian physicist, invented compound microscope in 1624 and named it "occhiolino" or "little eye". In 1625, his fellow academy member Giovanni Faber coined the word "microscope" for Galileo's invention. It won't be wrong to say that invention of compound microscope is a significant contribution to modern science.
Parts of a Compound Microscope
there are two components
: the structural components and the optical components.

10. Structural Components As the name suggests, these are the components that make up the configuration of a compound microscope. There are three basic structural components and they are as follows. Head or Body - Head represents the upper part, and it consists the main optical parts of a compound microscope. Base - Base is the bottom part, which supports the microscope. Illuminator is located in the base. Arm - Arm supports the head component and connects it to the base.

11. Optical Components Working of a microscope entirely depends on the optical components. The optical parts are affixed to the rigid arm of the microscope. Eyepiece - Eyepiece or oculars are secondary lenses located at the top, near the eye. It is through this lens that we view an object in a microscope. Mostly, their magnification is 10x. Objectives - Objectives are the primary lenses of a compound microscope. Typically, there are objective lenses of different powers in a compound microscope. Their magnifications ranges from 4x - 100x, which are labeled on their sides. Nosepiece - Nosepiece supports the objectives. The objectives are mounted on a rotating turret so that any of the objectives can be conveniently used for viewing. Coarse and Fine Adjustment Knobs - Coarse and fine adjustment knobs are used for focusing, so as to get a clear image of the object.

12. Stage - Stage is the platform, where the slide to be viewed is placed
Stage - Stage is the platform, where the slide to be viewed is placed. For higher magnification studies, mechanical stage is used for fine movement of the slide. Stage Clips - Stage clips are used to keep the slide in place, where there is no mechanical stage. Diaphragm - Diaphragm is a rotating disc located below the stage. It is used to control the amount of light reaching to the specimen. Condenser - Condenser collects light from the illuminator and focuses it onto the object. It is located below the stage and in association with the diaphragm. Body Tube - Body tube or eyepiece tube is the part that connects the oculars to the objectives. Thus, oculars are placed at the upper portion above the objectives. Illuminator - Illuminator is the source of light in a microscope. It is placed in the base component.

13. Diaphragm

14. Resolving power is the ability of an imaging device to separate (i. e
Resolving power is the ability of an imaging device to separate (i.e., to see as distinct) points of an object that are located at a small angular distance.