What is a Compound microscope? Applications of Compound microscope

Compound microscope

Microscopes are useful in all fields of science for a deep study of any specimen and a Compound microscope is one of the kind. It has proven useful to pharmaceutical companies, fiber industries, Biological study and Forensic investigations as well.

There are many different microscopes invented but the compound microscope is quite different and beneficial to scientists. It helps in obtaining high magnification of the samples up to 1000x as compared to a Simple microscope (100x magnification).

The compound microscope is one of the kind invented by Dutch lens maker Hans and his son Zacharias Janssen. It is a high magnifying microscope also known as a high power microscope or biological microscope that helps in the examination of minute specimens at high magnification which are not visible to the naked eye such as bacteria or virus.

Compound microscope has many properties that has made it superior over other microscopes. Here, in this article, we will tell you how a compound microscope works, and its applications in different fields.

What is a Compound microscope?

A compound microscope is a high magnifying microscope that is used to examine the smaller samples that are not visible to the naked eye such as diatoms, bacteria, or viruses. In this microscope, the sample is kept in a glass slide and is then kept on the stage of the microscope to examine through the lenses.

It is used to obtain a high magnification of samples from 10x-1000x which is possible with the combination of two lenses i.e., an ocular lens and an objective lens. The Ocular lens is in the eyepiece whereas the objective lens is the closest to the sample.

It is also known as a Common light microscope or Bright-field microscope because the image formed in a bright illuminated field.

Who invented Compound microscope?

The Compound microscope was invented in the late 16th century by Dutch lens-maker Hans and his son Zacharias Janssen. It was confirmed through a letter written by William Borelius, a Dutch physician and a childhood friend of Zacharias Janssen who wrote the specifics of the microscope developed by them.

Types of Compound microscopes

When we hear the term Compound microscope, we think of a biological microscope. It is true that a biological microscope is a compound microscope but there are many different types of compound microscopes which include a Phase-Contrast microscope, Polarizing microscope, Fluorescence microscope, and metallurgical microscope.

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A biological microscope can be termed as the brightfield or transmitted light microscope.

The phase-contrast microscope is a type of compound microscope that is used to examine blood and bacteria. It uses a phase condenser and an objective lens to bring out the contrast in a sample so that the image is formed without staining the sample.

A Polarizing microscope is used to examine minerals, rocks, chemicals, etc. In this microscope, an analyzer and a polarizer are used to cross-polarize the light and examine the colors in the optical path of light of the sample.

A metallurgical microscope has reflective light and is used for higher magnification and is used to examine the samples that do not let the light pass through them such as metals.

A fluorescence microscope is a kind of biological microscope which uses different wavelengths of light in order to examine a sample.

Instrumentation of Compound microscope

The parts of a compound microscope can be classified into Mechanical as well as optical parts.

Parts of compound microscope

Mechanical parts

  • Base: The support of the whole microscope.
  • Arm: An arm is a handle that holds and supports the whole microscope.
  • Stage: It is a place where the glass slides containing specimens are kept.
  • Stage Clips: These are placed over the stage to handle the glass slide for delicate movement of the slide.
  • Body tube: It is used to connect the eyepiece to the objective lenses.
  • Diaphragm: It is kept below the stage to adjust the intensity of light passing through the sample.

Optical Parts

  • Nose piece: is used to hold other objective lenses which are able to rotate while examination of an object.
  • Fine adjustment knob: It is used for accurate and sharp focusing of the specimen.
  • Coarse adjustment knob: It is used to move the body tube up and down such that the specimen comes up and down for proper focus.
  • Eyepiece:  It is the lens that is used to form an enlarged image of the object. The magnification of the lens can be 5x, 10x, or 15x.
  • Mirror: The mirror is attached to the lower end of the arm with a concave mirror on one side and plain on the other for reflection of light into the microscope.
  • Objective lens: These lenses have a magnification of 10x, 45x, and 100x which can be rotated during the examination and are connected to the revolving nosepiece. The 10x objective is known as Low power objective, 40x is known as the High dry objective and 100x is known as the Oil-immersion objective.
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Principle of Compound microscope

When a specimen is placed just beyond the focus of its objective lens, a virtual, erect, and highly magnified image of the sample is formed at the least distance of vision from the eye which is held closer to the eyepiece.

Working of Compound microscope

  • The specimen to be examined is kept on a clean glass slide and the slide is then placed between the condenser lens and the objective lens.
  • Apply the stage clips onto the slide to avoid any unnecessary and delicate movement of the slide.
  • The beam of light coming from the base of the microscope is focused through the condenser lens onto the specimen.
  • When the ray of light passes through the specimen to the objective lens, it forms an image inside the lens also known as the primary image. The image is then further magnified as the light passes through the ocular lens in the eyepiece.
  • For different magnifications use different objective lenses connected with the nosepiece.
  • Use the Coarse adjustment knob for shifting the stage upwards or downwards wherever the focus of the specimen is exact.
  • Use the fine adjustment knob for shifting the slider left or right to see the particular thing in the focused specimen.

Ray diagram of Compound microscope

Ray diagram of compound microscope
  • The specimen AB is placed just beyond the principal focus Fo‘ of the objective lens.
  • A ray of light AO from A goes parallel to the principal axis towards the objective lens and converges. Another ray of light goes through the optical center C1 of the objective lens and passes at a straight line to meet with the previous ray at A’B’ to form a primary image within the microscope.
  • This primary image is placed inside the microscope in such a way that it becomes the object for the eyepiece and is placed between the optical center C2 of the eyepiece and its principal focus Fe’.
  • Now both the rays coming from the objective lens are again refracted from the eyepiece and converges. Since both the rays are going away they cannot meet to form an image.
  • When produced backward both the rays meet at a point to form a magnified, virtual and erect image of the specimen visible to the eye.
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Magnification of Compound microscope

The magnification power of a compound microscope is given as:

M    =  D/ fo * L/fe   

where, M = Magnification power of compound microscope

D = Least distance of distinct vision (25 cm)

             L = Length of the microscope tube

             fo = Focal length of the objective lens

             fe = Focal length of the eye-piece lens

The magnifying power of a compound microscope increases when the focal lengths of the objective lens, as well as the eyepiece, are decreased. It can be clearly seen in the magnification power formula given above.

Applications of Compound microscope

  • It is used highly for the identification of diseases and blood analysis in pathology labs.
  • It is used for the detection of minerals and metals in human bodies for solving criminal cases.
  • It is helpful for students in schools and colleges to view bacteria and viruses which are invisible to the naked eye.
  • It is used by biologists for studying plant cells and micro-organisms.
  • It is used for the detection of drugs by Toxicologists and Forensic experts.
  • It is used for examining different kinds of biological & physical specimens in Forensic investigations.


Compound microscopes are very useful for students for their academic experiments and scientists for Researches due to its simple procedure, availability, and its advantages over others. Although it doesn’t have sufficient features but serves its purpose quite successfully to the users.

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