Microscope is a device used for the magnification of the sample. It is commonly used in laboratory for studying the specimen. In our previous session we have studied about various types of microscopes. In this session, let us study in detail about the compound microscope.
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What is a Compound Microscope?
A compound microscope is defined as
A microscope with a high resolution and uses two sets of lenses providing a 2-dimensional image of the sample.
The term compound refers to the usage of more than one lens in the microscope. Also, the compound microscope is one of the types of optical microscopes. The other type of optical microscope is a simple microscope. The difference between simple and compound microscope is that a simple microscope uses only one lens while the compound microscope uses more than one lens.
Diagram of Compound Microscope
It is said that the invention of the compound microscope was done by the Dutch spectacle-maker Zacharias Janssen in early 1590. However, there are no records that support that Zacharias Janssen used this microscope.
Working Principle of Compound Microscope
A compound microscope is considered to be one of the standard microscopes that can be used for general purposes. The arrangement of the lens is such that it magnifies the objects from the complex system.
There are two types of lenses that are used in the compound microscope:
- The objective lens is placed close to the object that needs to be examined.
- The eyepiece allows the image to be viewed. The eyepiece is also known as the ocular lens.
The light is made to pass through the thin transparent object. A magnified image of the object is obtained by the objective lens. This image is known as the real image. The eyepiece or the ocular lens then magnifies the real image more and is viewed as the virtual image. The compound microscope is also known as the bright-field microscope because the light passes directly through the light source to the eye through the two lenses. This mechanism makes the field of vision brightly illuminated.
Parts of Compound Microscope
The compound microscope is mainly used for studying the structural details of cell, tissue, or sections of organs. The parts of a compound microscope can be classified into two:
- Non-optical parts
- Optical parts
The base is also known as the foot which is either U or horseshoe-shaped. It is a metallic structure that supports the entire microscope.
The connection between the base and the arm are possible through the pillar.
The arm is also known as the limb which is a metallic handle forming the connection between the arm to the inclined joint. The stage and the body tube is supported by the arm.
If the observation has to be done in a sitting position, then the microscope can be tilted using the inclination joint.
It is the metallic platform that is fitted to the lower part of the arm with a hole in the center. The microscopic slides are placed on the stage either by using side clips or by mechanical stage clips.
The main purpose of the body tube is to hold the objective and ocular lenses at the two ends. The end where the ocular lens is present is known as the head while the end where the objective lens is placed is known as the nose piece. For the passage of light rays through the body tube, there is a pathway.
The upper end of the body tube has a small fixed tube which is known as the drawtube. The main function of the drawtube is to hold the ocular lens.
Rack and Pinion
To bring the object under focus, the rack and pinion are either attached to the body tube or the stage.
These are two pairs of adjusting screws that are used either for a coarse adjustment or for fine adjustment. When a fine adjustment is made, the body tube or the stage moves extremely short distances while in coarse adjustment, the body tube and stage move up. Through fine adjustment, a sharp image can be obtained.
The rack and pinion have a small screw that is used for stopping the downward sliding of the body tube. This prevents damage to the objective lens.
The amount of light falling on the object can be controlled through the diaphragm. It is present below the stage. The disc and iris are the two types of diaphragm.
It is present below the diaphragm. The focusing of light can be done by adjusting the condenser by moving it either up or down.
A reflector is a mirror that is attached above the base. One side of the mirror has a plane mirror while the other side has a concave mirror. When the light is strong, the plane mirror side is used and when the light is weak, the concave mirror side is used. The light on the object is directed with the help of the reflector through the diaphragm and condenser.
These lenses are present over the nose piece. There are two to three types of objective lenses:
- Low power
- High power
- Oil immersion
The objective lens is a compound lens that forms a real inverted image of the image inside the body tube.
The ocular lens is also known as the eyepiece. The image of microscopic objects can be viewed through these lenses. There are four types of magnification that can take place in the ocular lens:
The binocular head is the device that uses two eyepieces and has many mirrors and prisms, which makes the passage of light easier.
Advantages and Disadvantages of Compound Microscope
Advantages of Compound Microscope
- Due to the usage of multiple lenses, one can obtain detailed information about the sample.
- These microscopes have their own sources of light.
- This microscope is user-friendly and easy to handle.
Disadvantages of Compound Microscope
- The magnification of the sample is possible only to a certain extent, once this limit is reached the sample cannot be viewed.
Uses of Compound Microscope
- The identification of diseases becomes easy in pathology labs with the help of a compound microscope.
- Forensic laboratories use compound microscopes for the detection of human fingerprints.
- The presence of metals can be detected with the help of a compound microscope.
- The study of bacteria and viruses becomes easy with the help of a compound microscope.
- Schools use compound microscopes for academic purposes.
Read related concepts:
|What is simple microscope?|
|Uses of microscope|
|Difference between light microscope and electron microscope|
Frequently Asked Questions – FAQs
What is the difference between a stereomicroscope and a compound microscope?
The difference between a stereomicroscope and a compound microscope is that in the compound microscope there is a single path of light that travels in a way such that it splits before reaching the eyepiece. This is possible because of the number of ocular tubes in the microscope. This also means that the sample must be translucent enough so that the light passes through it and every eyepiece contains the same image.
Whereas in a stereo microscope there are two objective lenses that are used for magnification. Each lens captures the image individually, which means that there is a requirement of two paths of light for the creation of a 3D image of the sample.
What are zooming microscopes?
A zooming microscope is defined as a microscope that comes with a knob that is used for adjusting the magnification. These microscopes can be found in a medium and high-quality stereomicroscope as they make stereomicroscope more flexible during use.
What are fixed power microscopes?
A fixed power microscope is defined as a microscope that has a number of objectives but lacks a knob that can be used for magnification. Most of the compound microscopes have fixed power magnification.
What is the field of view of a microscope?
The field of view of a microscope is defined as how large the area is seen within the eyepiece. The lower the magnification, the smaller the image seen in the eyepiece. As the magnification increases, the size of the image gets bigger although the size of the hole in the eyepiece remains unchanged.
What is the depth of field of a microscope?
The depth of field of a microscope is defined as how deep the focus area is for the given values of magnification.
What is the focal distance of a microscope?
The focal distance of a microscope is defined as the distance at which the objective lens needs to be placed with respect to the sample in order to view a focused image.
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