Compound Microscope: Definition, Diagram, Parts and Uses

Compound Microscope

Edited By Vishal kumar | Updated on Jul 02, 2025 06:11 PM IST

A compound microscope is an essential tool in the field of biology, allowing us to explore the intricate details of microscopic organisms and cellular structures. This optical instrument uses a combination of lenses to magnify small objects, providing a detailed view that is impossible to achieve with the naked eye. In everyday life, compound microscopes play a crucial role in medical laboratories for diagnosing diseases, in forensic science for analyzing evidence, and in research facilities for advancing scientific knowledge. For instance, they enable scientists to study bacteria and viruses, leading to the development of vaccines and treatments that save millions of lives. By unveiling the hidden world of the microscopic, compound microscopes bridge the gap between the unseen and the understood, making them indispensable in both scientific discovery and practical applications. In this article we will discuss the concept of Compound microscope and some solved examples for better clarity.

This Story also Contains

What is a Compound Microscope?
Case 1: The final image is formed at D:
$m_D=\frac{L}{f_o}\left(1+\frac{D}{f_e}\right)$..
Case 2: The final image is formed at $\infty$ :
Magnification $m_{\infty}=\frac{v_0}{u_0} \cdot \frac{D}{f_e}$ and length of tube $L_{\infty}=v_0+f_e$ In terms of length $m_{\infty}=\frac{\left(L_{\infty}-f_o-f_e\right) D}{f_o f_e}$.
Solved Examples Based on Compound Microscope
Summary

Compound Microscope

What is a Compound Microscope?

A compound microscope is made up of two converging lenses called objective and eye lenses. It is used to view magnified images of small objects on a glass slide. It can achieve higher levels of magnification than stereo or other low-power microscopes and reduce chromatic aberration.

$f_{\text {eyclens }}>f_{\text {objective }} \text { and }(\text { diameter })_{\text {eyelens }}>(\text { diameter })_{\text {objective }}$
The intermediate image is real and enlarged. The final image is magnified, virtual and inverted.

Here in the diagram

$u_o=$ Distance of object from objective (o),
$v_o=$ Distance of image $\left(A^{\prime} B^{\prime}\right)$ formed by objective from objective,
$u_e=$ Distance of $A^{\prime} B^{\prime}$ from eye lens,
$v_\epsilon=$ Distance of final image from eye lens,
$f_o=$ Focal length of objective,
$f_e=$ Focal length of eye lens.

Here are two cases

Case 1: The final image is formed at D:

Magnification $m_D=\frac{v_o}{u_o}\left(1+\frac{D}{f_e}\right)$ and length of the microscope tube (distance between two lenses) is $L_D=v_e+u_e$
Generally, the object is placed very near to the principal focus of the objective hence $u_o \equiv f_o$ The eyepiece is also of small focal length and the image formed by the objective is also very near the eyepiece.

So $v_o \equiv L_D$ is the length of the tube.
Hence, we can write

$m_D=\frac{L}{f_o}\left(1+\frac{D}{f_e}\right)$..

Case 2: The final image is formed at $\infty$ :

Magnification
$m_{\infty}=\frac{v_0}{u_0} \cdot \frac{D}{f_e}$ and length of tube $L_{\infty}=v_0+f_e$
In terms of length $m_{\infty}=\frac{\left(L_{\infty}-f_o-f_e\right) D}{f_o f_e}$.

- For large magnification of the compound microscope, both $f_o$ and $f_e$ should be small.
- If the length of the tube of the microscope increases, then its magnifying power increases.
- The magnifying power of the compound microscope may be expressed as $M=m_o \times m_e$ where $m_0$ is the magnification of the objective and $m_e$ is magnifying the of eyepiece.

Solved Examples Based on Compound Microscope

Example 1: The separation L between the objective (f=0.5cm) and the eyepiece (f=5cm) of a compound microscope is 7cm. The angular magnification produced by this microscope when the eye is least strained is:

1) -5

2) -10

3) -15

4) -20

Solution:

Compound Microscope

$m=-\frac{v_o}{u_o} \cdot \frac{D}{u_e}$
wherein

$v_o$ and $u_o$ is the distance from the objective.
$u_e$ Distance from the eyepiece.

Maximum magnification
$
m=-\frac{v_o}{u_0}\left(1+\frac{D}{f_e}\right)
$

Since the eye is least strained hence the final image will form at infinity. In such a case, an image formed by the object should form at the focus of the eyepiece.

$
\begin{aligned}
& v_0=7 \mathrm{~cm}-5 \mathrm{~cm}=2 \mathrm{~cm} \quad f_0=0.5 \mathrm{~cm} \\
& \frac{1}{u}=\frac{1}{v_0}-\frac{1}{f_0}=\frac{1}{2}-\frac{1}{0.5}=\frac{-3}{2} \\
& \text { or }^u=\frac{-2}{3} \mathrm{~cm}
\end{aligned}
$

Angular magnification
$
m=\frac{v_0}{u_0} \cdot \frac{D}{f_e}=\frac{-25}{5}=-15 \mathrm{~cm}
$

Hence, the answer is the option (3).

Example 2: In a compound microscope, the focal length of the objective lens is 1.2 cm and the focal length of the eyepiece is 3.0 cm. When an object is kept at 1.25 cm in front of the objective, the final image is formed at infinity. The magnifying power of the compound microscope should be :

1) 200

2) 100

3) 400

4) 150

Solution:

Maximum magnification
$
m=-\frac{v_o}{u_0}\left(1+\frac{D}{f_e}\right)
$
$
\begin{aligned}
& \quad f_e=3 \mathrm{~cm} \\
& f_0=1.2 \mathrm{~cm} \\
& u_0=1.25 \mathrm{~cm}, v_e=\infty \\
& \Rightarrow \frac{1}{V_0}=\frac{1}{f_0}+\frac{1}{u_0}=\frac{1}{1.2}-\frac{1}{1.25} \\
& V_0=30 \mathrm{~cm} \\
& \qquad \frac{v_0}{u_0} \cdot \frac{D}{f_e}=\frac{30}{1.25} \times \frac{25}{3}=200 \\
& \text { }
\end{aligned}
$

Hence, the answer is the option (1).

Example 3: The focal length of the objective and the eyepiece of the compound microscope are 2cm and 3cm respectively. The distance between the objective and the eyepiece is 15 cm. The final image formed by the eyepiece is at infinity the distance (in cm) of the object and image produced by the objective, measured from the objective lens respectively.

1) 2.4 and 12

2) 2.4 and 15

3) 2.3 and 3

4) 2.3 and 12

Solution:

Length of the compound microscope
$
L=v_o+u_e
$
$v_o=$ Image distance from the objective.
$u_e=$ Object distance from the eyepiece
$
\begin{aligned}
& f_0=2 \mathrm{~cm} \quad f_e=3 \mathrm{~cm} \\
& \mathrm{I}=15 \mathrm{~cm}
\end{aligned}
$

The final image is formed at infinity hence, the image formed by the objective is at the focal point of the eyepiece

$
\begin{aligned}
& u_e=f_e=3 \mathrm{~cm} \\
& v_o=l-f_e=12 \mathrm{~cm}
\end{aligned}
$

For objective:
$
\begin{array}{ll}
v_o=12 c m & f_o=2 \mathrm{~cm} \\
\frac{1}{v_0}-\frac{1}{u_0}=\frac{1}{f_0} & \\
\frac{1}{12}-\frac{1}{u_0}=\frac{1}{2} &
\end{array}
$
or
$
\frac{1}{u_0}=\frac{1}{12}-\frac{1}{2}=\frac{1-6}{12}
$
or,
$
u_0=-2.4 \mathrm{~cm} \quad v_0=12 \mathrm{~cm}
$

Hence, the answer is the option (1).

Example 4: If we used a magnification of 375 from a compound microscope of tube length 150mm and an objective of focal length 5mm, the focal length of the eyepiece should be close to :

1) 12mm

2) 33mm

3) 22mm

4) 2mm

Solution:

The magnification is given by
$
\begin{aligned}
& M=\frac{L}{f_0}\left(1+\frac{D}{f_c}\right) \\
& \Rightarrow 375=\frac{150}{5}\left(1+\frac{25}{f_e}\right) \\
& \Rightarrow f_e=22 \mathrm{~mm}
\end{aligned}
$

Hence the option (3) is correct.

Example 5: A compound microscope consists of an objective lens of focal length 1cm and an eyepiece of focal length 5 cm with a separation of 10 cm. The distance between an object and the objective lens, at which the strain on the eye is minimum is $\frac{n}{40} \mathrm{~cm}$ The value of n is ______.

1) 50

2) 100

3) 150

4) 200

Solution:

Image by objective is formed at the focus of the eye-piece
$\therefore$ For objective, $v=5, u, f=1 \mathrm{~cm}$
$
\begin{array}{r}
\frac{1}{5}-\frac{1}{u}=\frac{1}{1} \Rightarrow \frac{1}{5}-1=\frac{1}{u} \\
\therefore \quad|u|=\frac{5}{4} \mathrm{~cm} \Rightarrow|u|=\frac{50}{40} \mathrm{~cm} \\
\therefore \quad n=50
\end{array}
$

Summary

A compound microscope, utilizing two converging lenses—objective and eyepiece—provides high magnification of small objects. The final image produced is magnified, virtual, and inverted. For maximum magnification, the focal lengths of both lenses should be small, and the microscope's tube length should be optimal. Solved examples illustrate the calculations for determining magnification and focal lengths in different scenarios, emphasizing the importance of precise measurements for accurate results.

Frequently Asked Questions (FAQs)

1. How does the thickness of the cover slip affect image quality in microscopy?

Cover slip thickness is critical because objectives are designed to work with a specific cover slip thickness (usually 0.17 mm). Using a cover slip that's too thick or thin can introduce spherical aberration, reducing image quality. This effect is more pronounced at higher magnifications. Some high-end objectives have correction collars to compensate for variations in cover slip thickness.

2. What is the difference between a finite and an infinity-corrected optical system in microscopes?

In a finite optical system, the objective forms a real image at a fixed distance (usually 160mm) inside the microscope tube. Infinity-corrected systems use objectives that form their image at infinity, with an additional lens (called a tube lens) to form the real image. Infinity systems allow for easier insertion of accessories like filters without affecting image quality and reduce the impact of tube length variations.

3. How does chromatic aberration affect image quality in a compound microscope?

Chromatic aberration occurs because different wavelengths of light are refracted to different degrees by a lens. This can cause color fringes around the edges of objects in the image. Modern microscopes use achromatic or apochromatic lenses to correct for this aberration, ensuring that different colors focus at the same point and producing sharper, more accurate images.

4. What is the difference between depth of field and depth of focus in microscopy?

Depth of field refers to the thickness of the specimen that appears in focus at one time. It decreases as magnification increases. Depth of focus, on the other hand, is the distance the image plane can be moved while maintaining acceptable focus. Understanding these concepts is important for proper focusing and interpreting three-dimensional structures in microscopic specimens.

5. What is spherical aberration, and how does it affect image quality in a compound microscope?

Spherical aberration occurs when light rays passing through the outer edges of a lens focus at a different point than rays passing through the center. This results in a blurred or hazy image, especially at the edges of the field of view. It's more pronounced at higher magnifications and can be minimized by using corrected lenses, proper cover slip thickness, and immersion techniques for high-power objectives.

6. How does changing the objective lens affect the field of view?

Changing the objective lens affects the field of view inversely to the magnification. As you switch to a higher magnification objective, the field of view becomes smaller. This means you can see more detail, but in a smaller area of the specimen. Conversely, lower magnification objectives provide a larger field of view but less detail.

7. How is the total magnification of a compound microscope calculated?

The total magnification of a compound microscope is calculated by multiplying the magnification of the objective lens by the magnification of the eyepiece. For example, if you're using a 40x objective lens and a 10x eyepiece, the total magnification would be 40 x 10 = 400x.

8. What is the role of the eyepiece (ocular lens) in a compound microscope?

The eyepiece, or ocular lens, further magnifies the real image produced by the objective lens. It creates a virtual image that appears larger to the observer's eye. The eyepiece typically provides an additional 10x magnification, which is multiplied by the objective lens magnification to give the total magnification.

9. Why is proper focusing important in using a compound microscope?

Proper focusing is crucial because compound microscopes have a very shallow depth of field, especially at higher magnifications. This means only a thin slice of the specimen is in focus at any given time. Accurate focusing ensures that the part of the specimen you want to observe is sharp and clear, allowing you to see important details and structures.

10. What is the working distance of an objective lens, and why is it important?

The working distance is the space between the front of the objective lens and the top of the cover slip when the specimen is in focus. It's important because it determines how close you can get to the specimen without touching it. Higher magnification objectives generally have shorter working distances, which can make focusing more challenging and increase the risk of damaging the lens or specimen.

11. What is a compound microscope and how does it differ from a simple microscope?

A compound microscope uses multiple lenses to magnify an object, typically an objective lens and an eyepiece. It differs from a simple microscope, which uses only a single lens. Compound microscopes can achieve much higher magnifications and are used for observing very small specimens like cells and microorganisms.

12. What are the main components of a compound microscope?

The main components of a compound microscope include:

13. Why does a compound microscope invert the image of the specimen?

A compound microscope inverts the image because of the way light rays pass through the objective lens. The objective lens forms a real, inverted image, which is then magnified by the eyepiece. This double inversion results in a final image that appears upside-down and reversed left-to-right compared to the actual specimen.

14. What is the function of the objective lenses in a compound microscope?

Objective lenses are the primary magnifying lenses in a compound microscope. They collect light from the specimen and form a magnified real image. Objectives come in different magnifications (e.g., 4x, 10x, 40x, 100x) and are responsible for the initial enlargement of the specimen's image.

15. How does a compound microscope produce a magnified image?

A compound microscope produces a magnified image in two stages. First, the objective lens creates a magnified real image of the specimen. Then, the eyepiece further magnifies this image, producing a virtual image that appears larger to the observer's eye. This two-stage magnification allows for much greater total magnification than a single lens could achieve.

16. How does oil immersion work in high-power microscopy?

Oil immersion involves placing a drop of special immersion oil between the specimen and a high-power objective lens (usually 100x). The oil has a refractive index similar to glass, which reduces light refraction and allows more light to enter the objective. This technique increases resolution and image clarity at very high magnifications.

17. How does the numerical aperture (NA) of an objective lens affect image quality?

The numerical aperture is a measure of an objective's ability to gather light and resolve fine specimen detail. Higher NA values result in better resolution and brighter images. However, higher NA objectives also have shorter working distances and are more sensitive to errors in cover slip thickness and immersion technique.

18. What causes spherical aberration in a compound microscope, and how can it be minimized?

Spherical aberration occurs when light rays passing through the outer edges of a lens focus at a different point than rays passing through the center. This can cause blurring and reduced image quality. It can be minimized by using high-quality, corrected lenses, ensuring proper cover slip thickness, and using immersion oil with high-power objectives.

19. What is the difference between bright field and dark field microscopy?

Bright field microscopy is the standard technique where light passes directly through the specimen, creating a bright background with darker objects. Dark field microscopy uses a special condenser to block direct light, allowing only scattered light from the specimen to reach the objective. This creates a dark background with bright objects, enhancing contrast for transparent specimens.

20. How does phase contrast microscopy work, and when is it useful?

Phase contrast microscopy converts differences in the phase of light waves passing through a specimen into differences in brightness. It uses a special condenser and phase plate to create contrast in transparent specimens without staining. This technique is particularly useful for observing living cells and other unstained, transparent biological specimens.

21. What is the resolution limit of a light microscope, and what determines it?

The resolution limit of a light microscope is the smallest distance between two points that can be distinguished as separate. It's determined by the wavelength of light used and the numerical aperture of the objective lens. The theoretical limit is about 0.2 micrometers (200 nanometers) using visible light. This limit is due to the wave nature of light and is known as the diffraction limit.

22. How does Köhler illumination improve image quality in a compound microscope?

Köhler illumination is a technique that provides even, glare-free illumination of the specimen. It involves aligning and focusing the illumination system so that the light source is not directly imaged in the same plane as the specimen. This results in improved contrast, reduced glare, and better overall image quality, especially for high-magnification observations.

23. How does the refractive index of the mounting medium affect microscope performance?

The refractive index of the mounting medium (the substance in which the specimen is embedded) affects how light travels through the sample. If there's a large difference between the refractive indices of the specimen and the mounting medium, it can cause unwanted refraction and reduce image quality. Matching the refractive indices as closely as possible helps to minimize these effects and improve image clarity.

24. How does polarized light microscopy work, and what are its applications?

Polarized light microscopy uses polarizing filters to manipulate the vibration of light waves. It involves placing one polarizer below the specimen and another (called an analyzer) above it. This technique is useful for studying materials with birefringent properties, such as crystals, fibers, and stressed plastics. It can reveal structural details and compositions not visible with standard bright-field microscopy.

25. How does fluorescence microscopy work, and what are its advantages?

Fluorescence microscopy uses specific wavelengths of light to excite fluorescent molecules in a specimen, causing them to emit light of a longer wavelength. This technique allows for highly specific labeling of cellular components and can detect very low concentrations of fluorescent molecules. It's particularly useful for studying the localization and dynamics of specific molecules within cells.

26. How does immersion oil improve the resolution of a microscope?

Immersion oil improves resolution by increasing the numerical aperture of the objective lens. It fills the space between the front lens of the objective and the cover slip with a medium that has a refractive index similar to glass. This reduces the refraction of light rays as they pass from the specimen to the objective, allowing more light to enter the lens and improving both resolution and brightness of the image.

27. How does the working principle of a DIC (Differential Interference Contrast) microscope differ from a standard bright-field microscope?

DIC microscopy uses polarized light and a special prism to create contrast in transparent specimens. It splits the light beam into two rays that pass through the specimen slightly offset from each other. When recombined, these rays interfere, creating a pseudo-3D image with enhanced contrast. This technique is excellent for observing unstained living cells and other transparent specimens, revealing details not visible in bright-field.

28. How does the wave nature of light limit the resolution of optical microscopes?

The wave nature of light imposes a fundamental limit on the resolution of optical microscopes, known as the diffraction limit. When light passes through a small aperture (like a lens), it diffracts and spreads out. This spreading means that a point source of light will always appear as a small disc (Airy disc) rather than a perfect point. The size of this disc determines the smallest distance at which two points can be distinguished, limiting the microscope's resolution.

29. What is the function of the diaphragm in a compound microscope?

The diaphragm controls the amount of light that reaches the specimen. It's an adjustable aperture that can be opened or closed to increase or decrease illumination. This helps to optimize contrast and clarity in the image, especially when viewing specimens with different levels of transparency or opacity.

30. What is the difference between coarse and fine focus knobs?

Coarse focus knobs make large adjustments to the distance between the objective lens and the specimen, allowing for rapid focusing. Fine focus knobs make very small adjustments, allowing for precise focusing once you're close to the correct focal plane. Using both in sequence helps achieve optimal focus quickly and accurately.

31. What is the purpose of a condenser lens in a compound microscope?

The condenser lens focuses light onto the specimen, controlling the illumination of the sample. It helps to maximize resolution and contrast by ensuring that light rays converge at the proper angle to illuminate the specimen evenly. A well-adjusted condenser is crucial for achieving optimal image quality, especially at higher magnifications.

32. What is the difference between parfocal and non-parfocal objective lenses?

Parfocal objective lenses are designed so that when you switch between different magnifications, the specimen remains approximately in focus. This saves time and reduces the risk of damaging lenses or specimens. Non-parfocal objectives require refocusing each time you change magnification. Most modern compound microscopes use parfocal objectives for convenience and safety.

33. What is the purpose of the stage clips on a compound microscope?

Stage clips hold the microscope slide in place on the stage. They prevent the slide from moving during observation, which is crucial for maintaining focus and examining specific areas of the specimen. Proper use of stage clips ensures that the specimen remains stable, especially when using higher magnifications where even small movements can cause the image to blur or shift out of view.

34. What is the function of the iris diaphragm in a compound microscope?

The iris diaphragm, located in the condenser, controls the angle of the light cone that illuminates the specimen. Adjusting the iris diaphragm affects both the amount of light and the resolution of the image. Opening the diaphragm increases resolution but decreases contrast, while closing it increases contrast but can reduce resolution. Finding the optimal setting is crucial for balancing these factors.

35. How does the numerical aperture (NA) of an objective lens relate to its resolving power?

The numerical aperture of an objective lens is directly related to its resolving power. Higher NA values allow the lens to capture more light, including light diffracted at higher angles from the specimen. This increases the lens's ability to resolve fine details. The relationship is described by the formula: Resolution = λ / (2 × NA), where λ is the wavelength of light. Thus, higher NA values result in better resolution.

36. What is the purpose of the parfocal distance in a compound microscope?

The parfocal distance is the distance from the nosepiece to the focal plane of the specimen. In a properly adjusted microscope, this distance remains constant when switching between objective lenses. This feature allows users to change magnification without completely refocusing, saving time and reducing the risk of damaging lenses or specimens by accidental contact.

37. What is the difference between resolution and magnification in microscopy?

Resolution is the ability to distinguish between two closely spaced objects as separate entities. Magnification is simply the degree to which an object appears enlarged. Increasing magnification doesn't necessarily improve resolution; it can make an image larger without revealing more detail. Resolution is limited by factors like the wavelength of light and the numerical aperture of the lens, while magnification can be increased indefinitely.

38. How does the refractive index of the specimen affect image formation in a compound microscope?

The refractive index of the specimen affects how light bends as it passes through. If there's a significant difference between the refractive indices of the specimen and its surrounding medium, it can cause distortions in the image. This is why matching the refractive index of the mounting medium to that of the specimen can improve image quality. Additionally, the contrast in the image is often due to differences in refractive index within the specimen.

39. What is the function of the mechanical stage on a compound microscope?

The mechanical stage allows precise and controlled movement of the microscope slide in the X and Y directions (horizontally and vertically in the field of view). It typically has calibrated scales that enable the user to record the position of specific features in the specimen. This is crucial for systematic examination of slides, especially when working with high magnifications where the field of view is very small.

40. How does the thickness of a specimen affect its visibility under different magnifications?

The thickness of a specimen becomes increasingly important at higher magnifications due to the decreasing depth of field. Thicker specimens may require focusing through multiple planes to observe all structures clearly. At high magnifications, only a thin slice of the specimen is in focus at any given time. This is why specimen preparation techniques like sectioning are often used to create very thin samples for high-magnification microscopy.

41. What is the purpose of the substage condenser in a compound microscope?

The substage condenser focuses light onto the specimen, controlling both the intensity and angle of illumination. It's crucial for achieving optimal resolution and contrast, especially at higher magnifications. A properly adjusted condenser ensures that the specimen is evenly illuminated and that the full numerical aperture of the objective lens is utilized, maximizing resolution and image quality.

42. How does the concept of conjugate planes apply to image formation in a compound microscope?

Conjugate planes in a microscope are pairs of planes that are optically related, where one plane contains the image of the other