Compound Microscope – Principle, Parts, Diagram Definition, Application

Compound Microscope is an optical instrument in which two lenses are used to magnify the small specimen. It is also called light microscope.

The two lenses are objective lens and eyepiece lens. The objective lens is near the specimen and eyepiece lens is near the eye.

The specimen is placed on glass slide. Light is passed through the specimen. For this, specimen must be thin and transparent.

The objective lens first magnifies the specimen. Then this image is again magnified by eyepiece lens. The final image is enlarged and inverted.

The magnification is generally 40X to 1000X. In advanced microscope it may be more. The image clarity depends on light, lens and proper focusing.

Stain and dye are used to make the specimen clear. Sometimes bright-field and phase contrast method are also used.

It is used to observe cells, bacteria, microorganisms, tissue structure and other small objects. It is used in biology, medicine, microbiology, pathology, laboratory and classroom study.

The first compound microscope was made around 1590 by Zacharias Janssen and Hans Janssen. Robert Hooke used compound microscope and wrote Micrographia in 1665. He used the term cell for cork pores.

Later Joseph Jackson Lister improved the lens system in 1830s. After that better glass, oil immersion lens and illumination method made the microscope more useful.

Working Principle of Compound Microscope

Principle of Compound Microscope is based on the principle of two stage magnification by using two lenses and light source.

In this microscope, light from the illuminator passes through the condenser. Then light passes through the thin specimen which is placed on the stage.

The objective lens is present very close to the specimen. It collects the light coming from the specimen and forms first magnified image. This image is real and inverted.

This first image acts as the object for the eyepiece lens or ocular lens. The eyepiece again magnifies this image and forms the final image.

The final image is highly magnified and virtual. It is seen by the observer through the eyepiece.

The total magnification of compound microscope is obtained by multiplying the magnification of objective lens and eyepiece lens.

The reaction is as follows-

Total magnification = Magnification of objective lens × Magnification of eyepiece lens

Magnification Power of Compound Microscope

Magnification power of compound microscope means total enlargement power of compound microscope. It is due to two lenses. One is objective lens and another is eyepiece lens.

Objective lens is near the object. Eyepiece lens is near the eye. First magnification is done by objective lens. Again magnification is done by eyepiece lens.

So, total magnification is multiplication of both lens power.

The formula is as follows-

Total magnification = power of objective lens × power of eyepiece lens

Example-

When 10X eyepiece and 40X objective are used,

Total magnification = 10X × 40X

= 400X

Thus the object is seen 400 times enlarged.

In compound microscope the objective lenses are commonly 4X, 10X, 40X and 100X. When 10X eyepiece is used, the total magnification becomes 40X, 100X, 400X and 1000X.

For higher magnification oil immersion lens is used. It reduces bending of light. So image becomes clear. It is used for observing bacteria and very small cell.

Parts of Compound Microscope

The parts of compound microscope are divided into two groups-

1. Structural parts
2. Optical parts

Structural parts

1. Base or foot – It is the lowermost part of the microscope. It supports the whole microscope and the light source is present in it.
2. Arm – It is the curved vertical part of microscope. It connects the base with head and also used for carrying the microscope.
3. Body tube or head – It is the hollow tubular part. It connects the eyepiece lens with the objective lenses.
4. Stage – It is a flat platform. The slide containing specimen is placed on it.
5. Stage clips – These are small metallic clips present on the stage. It holds the slide firmly in place.
6. Coarse adjustment knob – It is a large knob. It is used for rapid movement of stage for rough focusing.
7. Fine adjustment knob – It is a small knob. It is used for minute adjustment to get sharp and clear image.
8. Revolving nosepiece or turret – It is a rotating circular part present below the body tube. It holds the objective lenses and helps to change magnification.
9. Rack stop – It is a safety part. It prevents the stage from moving too much upward and protects the slide and lens.
10. Inclination joint – It is the joint by which microscope can be tilted backward for comfortable viewing.

Optical parts

1. Eyepiece or ocular lens – It is present at the top of microscope. It is used to observe the image and usually magnifies 10X or 15X.
2. Objective lenses – These are present near the specimen and attached to revolving nosepiece. Common objectives are 4X, 10X, 40X and 100X.
3. Illuminator or mirror – It is present at the base. It provides light and sends the light through the specimen.
4. Condenser lens – It is present below the stage. It collects and concentrates light on the specimen.
5. Diaphragm or iris – It is present below the stage. It controls the amount of light passing through the specimen.

Factors Play an Important Role in Magnification of Compound Microscope

The following are the important factors which affect the magnification of compound microscope–

1. Power of objective lens – The objective lens is present near the specimen. It gives first enlargement of the object. Common objective powers are 4X, 10X, 40X and 100X.
2. Power of eyepiece lens – The eyepiece lens is present near the eye. It magnifies the image again which is formed by objective lens. Its power is generally 5X to 30X.
3. Focal length of objective lens – The magnification depends on the focal length of objective lens. When the focal length is short, the magnification becomes high.
4. Focal length of tube lens – The tube lens also affect the magnification in special optical system. It works with the objective lens and helps to form total magnification.
5. Use of immersion medium – Immersion oil is used with high power objective lens. It reduces bending of light and helps to see clear image at high magnification. By this the microscope can show up to about 2000X magnification.
6. Combination of lenses – The total magnification depends on both objective lens and eyepiece lens. So, proper combination of both lens is important for getting required magnification.

Operating Procedure of Compound Microscope

The following are the operating procedure of compound microscope–

1. First the lenses, mirror, stage and slide are cleaned properly. Dust should not be present on them.
2. The eyepiece is observed and the mirror or light source is adjusted. Sufficient light should pass through the microscope.
3. The specimen slide is placed in the middle of the stage. It is fixed with the help of stage clips.
4. The nosepiece is rotated and the lowest power objective lens is brought in line with the specimen.
5. By looking from the side, the coarse adjustment knob is turned. The objective lens is brought near the specimen but it should not touch the slide.
6. The eyepiece is observed and the coarse adjustment knob is slowly turned upward. The image is brought into rough focus.
7. The fine adjustment knob is turned slowly. The object is made sharp and clear.
8. After focusing in low power, the nosepiece is rotated. The high power objective lens is brought in position to see fine details.
9. In high power objective, only fine adjustment knob is used. Coarse adjustment knob should not be used because it may break the slide.

Compound Microscope Resolution Power (Resolving Power)

Resolving power is the ability of compound microscope to distinguish two very close points as separate points. It is also called resolution. It is not same with magnification. Magnification only increases the size of image.

Resolving power is measured by the minimum distance between two points. This minimum distance is called d_min. If two points are closer than d_min, then they are seen as one point. If d_min is low, then resolving power is high.

The resolution depends on the wavelength of light (λ) used in illumination. Shorter wavelength gives better resolution. So blue light gives better resolution than red light.

It also depends on numerical aperture (NA) of the objective lens and condenser. Numerical aperture depends on the refractive index of medium and the half angle of cone of light entering the objective lens.

According to Abbe’s criterion, the minimum distance is-

d_min = λ / 2 n sin θ

Resolving power is-

Resolving power = 1 / d_min

= 2 n sin θ / λ

Here, λ is wavelength of light. n is refractive index of medium. θ is half angle of cone of light. When n sin θ increases, then resolution also increases.

Immersion oil is used between the specimen and objective lens. It increases refractive index of medium. So numerical aperture increases and resolving power is improved.

In light microscope, resolution has a fixed limit due to diffraction of light. Even with good lenses, two points below about 200-250 nm or 0.2-0.25 µm cannot be seen separately.

The lateral resolution is better than axial resolution. Lateral resolution means resolution in horizontal direction. Axial resolution means resolution in depth direction along the optical axis.

Lens defects, aberration, poor contrast and improper illumination decrease the resolving power. So the actual resolution becomes less than theoretical value.

Resolving power is more important than only magnification. High magnification without resolving power gives enlarged but blurred image.

Applications of Compound microscope

The following are the applications of compound microscope–

• In biology laboratory, compound microscope is used for the study of cells, tissues, bacteria and protozoa.
• In medical laboratory, it is used for examination of blood sample, tissue section and clinical sample.
• In pathology, it is used for observing diseased cell and abnormal tissue.
• In microbiology, it is used for studying bacteria, fungi, protozoa and other minute organisms.
• In forensic science, it is used for the examination of hair, fibre, soil particle, paint particle and small evidence.
• In document examination, it is used for observing ink mark, paper fibre and forged document.
• In educational laboratory, it is used for practical study of microscopic objects.
• In material science, it is used for observing metal, plastic and polymer surface.
• In industry, it is used for checking small crack, surface defect and quality of material.
• In environmental study, it is used for examination of water sample and soil sample.
• In botany, it is used for studying plant cell, plant tissue, algae and plant disease causing organism.

Advantages of Compound Microscopes

The following are the advantages of compound microscopes–

• Compound microscope has high magnification power due to the presence of objective lens and eyepiece lens.
• It can magnify very small objects like cells and bacteria up to about 1000X.
• It gives clear and sharp image when the lenses are properly focused.
• It has its own light system, so the object can be seen without depending only on outside light.
• The amount of light can be controlled by condenser and diaphragm.
• It is useful in many fields like biology, medicine, forensic science and material study.
• It is smaller and lighter than electron microscope.
• It can be easily carried from one place to another place.
• It is less costly than advanced microscopes like electron microscope.
• It is suitable for school, college, laboratory and general research work.
• It is simple to use, so students and beginners can also operate it easily.

Limitations Compound Microscope

The following are the limitations of compound microscopes–

• Compound microscope has limited magnification power. Generally it can magnify up to 1000X to 2000X only.
• Its resolving power is also limited. Objects smaller than about 200 nm cannot be clearly separated.
• It cannot be used for observing very small particles like viruses.
• The specimen must be very thin and transparent. Thick specimen cannot be observed properly.
• It gives only two dimensional image. So surface structure in 3D form cannot be studied.
• At high magnification, the depth of field becomes very low. So thick objects are difficult to focus.
• Many specimens need staining for clear observation. During staining, living specimens are usually killed.
• Proper skill is needed for focusing, light adjustment and placing of slide.
• Dust on lens, slide or mirror can disturb the clear image.
• Vibration of table or surrounding also affects the image formation.
• It depends on proper light. Without suitable light, the image is not clearly visible.

Stereomicroscope vs Compound microscope

The following are the difference between stereomicroscope and compound microscope–

Difference between compound and simple microscope

References

1. Ockenga, W. (2015, September 8). A brief history of light microscopy. Leica Microsystems.
2. Whipple Museum of the History of Science. (n.d.). A brief history of the microscope. University of Cambridge.
3. Mitutoyo. (n.d.). An in-depth guide to learning about different types of microscopes and choosing the right microscope for your application. Mitutoyo America Corporation.
4. Microscope World. (n.d.). Compound microscope parts.
5. Microscope.com. (n.d.). Compound microscope parts | Key components explained.
6. Allen Career Institute. (n.d.). Compound microscope – Features, uses & working principle.
7. ALMICRO Instruments. (n.d.). Compound microscope: Parts, diagram and working.
8. Motic America. (2021, June 15). Digital vs compound microscopes: Key differences & benefits. Motic Microscopes.
9. Chanda, C. (2025, March 27). What is the difference between digital microscope and traditional optical microscope? PreciPoint.
10. Joy, T. (2024, January 16). Digital vs. optical microscopes: An in-depth comparison. Evident Scientific.
11. ALMICRO. (2025, November 5). A complete guide to the compound microscope and its types.
12. National Center for Biotechnology Information. (n.d.). From animaculum to single molecules: 300 years of the light microscope. PubMed Central.
13. Alvarenga, L. (2022, February 3). How to align Köhler illumination in 6 simple steps. Evident Scientific.
14. Wilson, M. (2017, August 17). Koehler illumination: A brief history and a practical set up in five easy steps. Leica Microsystems.
15. Light Microscopy Core Facility. (n.d.). Kohler illumination procedure. Duke University.
16. Park, B. (n.d.). Magnification, numerical aperture (NA) and resolution in microscopic imaging. Basler AG.
17. Home Science Tools. (n.d.). Microscope forensics.
18. Microscope World. (n.d.). Microscope maintenance tips.
19. Microscope World. (n.d.). Microscope parts and specifications.
20. Microscope.com. (n.d.). Microscope maintenance.
21. ZEISS Microscopy. (2025, July 22). Microscope cleaning & maintenance: The complete guide for optimal performance. ZEISS.
22. Windelspecht, M. (2024). Microscopes in forensic investigations. EBSCO Research Starters.
23. Mualla, F., Aubreville, M., & Maier, A. (2018). Microscopy. In A. Maier, S. Steidl, V. Christlein, et al. (Eds.), Medical imaging systems: An introductory guide (Chapter 5). Springer.
24. Motic America. (2025, June 6). Microscopy 101: Understanding magnification, resolution, and lenses. Motic Microscopes.
25. Abramowitz, M., & Davidson, M. W. (n.d.). Numerical aperture and resolution. Evident Scientific.
26. BYJU’S. (n.d.). Parts of compound microscope.
27. Mokobi, F. (2025, April 9). Parts of a microscope with functions and labeled diagram. Microbe Notes.
28. Davidson, M. W. (n.d.). Resolution. Nikon’s MicroscopyU.
29. CLYTE Technologies. (2025, November 28). Step-by-step set up for perfect Koehler illumination for microscopy.
30. The compound microscope: An exhaustive analysis of optical principles, structural architecture, and scientific implementation. (n.d.).
31. EUROIMMUN UK. (2024, December 9). The evolution of microscopy: A glimpse through the lens. EUROIMMUN.
32. Science Museum. (n.d.). The microscope.
33. AmScope. (2025, April 28). The ultimate guide to microscope cleaning & maintenance.
34. Khot, S. (n.d.). The vital role of microscopes in forensic science. JSPM University.
35. Lyons, K. (2019, November 11). Use, care, and maintenance of a microscope. Medicanix Inc.
36. Kentfaith. (2025, October 12). What is the use of compound microscope? K&F Concept.
37. SISCO. (2026, February 25). What are the advantages of digital microscope?.