Compound Microscope- Definition, Labeled Diagram, Principle, Parts, Uses

A microscope is an instrument that allows us to see objects that are too small to be seen by the naked eye. Microscopes can be classified into different types based on their source of illumination, their mechanism of image formation, and their number of lenses. One of the most common and widely used types of microscopes is the compound microscope.

A compound microscope is a type of optical microscope that uses visible light and a system of lenses to magnify images of small subjects. The term "compound" refers to the fact that this microscope has more than one lens. In fact, a compound microscope consists of two main optical parts: the objective lens and the ocular lens.

Compound microscopes have a combination of lenses that enhances both magnifying powers as well as the resolving power. The resolving power is the ability to distinguish two adjacent points as separate entities. The magnifying power is the ratio of the size of the image to the size of the object.

The specimen or object, to be examined is usually mounted on a transparent glass slide and positioned on the specimen stage between the condenser lens and objective lens.

A beam of visible light from the base is focused by a condenser lens onto the specimen. The condenser lens also controls the amount and angle of light that reaches the specimen.

The objective lens picks up the light transmitted by the specimen and creates a magnified image of the specimen called the primary image inside the body tube. This image is inverted (upside down) and real (can be projected on a screen).

The primary image is again magnified by the ocular lens or eyepiece. The eyepiece acts like a simple magnifying glass and produces a secondary image that is virtual (cannot be projected) and enlarged.

The secondary image is what we see when we look through the eyepiece. It is also inverted, so we need to move the slide in the opposite direction of where we want to see.

When higher magnification is required, the nose piece is rotated after low power focusing to bring the objective of a higher power (generally 45X) in line with the illuminated part of the slide.

Occasionally very high magnification it required (e.g. for observing bacterial cell). In that case, an oil immersion objective lens (usually 100X) is employed. Oil immersion lenses use a drop of oil between the slide and the lens to reduce light refraction and increase resolution.

The common light microscope is also called a bright-field microscope because the image is produced amidst a brightly illuminated field. The image appears darker because the specimen or object is denser and somewhat opaque than the surroundings. Part of the light passing through or object is absorbed.

Magnification is the process of enlarging the apparent size of an object or image. In a compound microscope, magnification is achieved by the combined action of two sets of lenses: the objective lens and the eyepiece lens.

The objective lens is the one closest to the specimen, and it creates an enlarged image of the specimen called the primary image. The eyepiece lens is the one through which the viewer observes the specimen, and it further magnifies the primary image to produce the final image.

The total magnification of a compound microscope is calculated by multiplying the magnification of the objective lens by the magnification of the eyepiece lens. For example, if the objective lens has a magnification of 40x and the eyepiece lens has a magnification of 10x, then the total magnification is 40x * 10x = 400x. This means that the specimen appears 400 times larger than its actual size.

However, magnification alone does not determine the quality of the image. Another important factor is resolution, which is the ability to distinguish two points as separate entities. Resolution depends on the wavelength of light used and the numerical aperture of the objective lens. The numerical aperture is a measure of how much light the objective lens can gather from the specimen.

The higher the numerical aperture, the better the resolution. However, there is a limit to how much resolution can be achieved with visible light, which has a wavelength of about 400-700 nanometers. According to Abbe`s diffraction limit formula, the maximum resolution that can be obtained with visible light is about 0.2 micrometers (or 200 nanometers).

To overcome this limit, some compound microscopes use oil immersion objectives, which have a very high numerical aperture. Oil immersion objectives are designed to be used with a drop of oil between the objective lens and the specimen slide. The oil has a similar refractive index as glass, which reduces the loss of light due to refraction and increases the effective numerical aperture.

Oil immersion objectives can achieve a resolution of about 0.1 micrometers (or 100 nanometers), which is twice as good as dry objectives. However, oil immersion objectives also have some disadvantages, such as requiring careful cleaning and alignment, and being incompatible with some staining techniques.

Therefore, when choosing a compound microscope, one should consider both magnification and resolution, as well as other factors such as contrast, depth of field, field of view, and cost. A compound microscope can provide a detailed view of microscopic structures and phenomena, but it also has its limitations and challenges.

A compound microscope consists of several parts that work together to produce a magnified image of a specimen. The main parts of a compound microscope are:

• The Eyepiece and body tube: The eyepiece is the lens that you look through to see the magnified image. It usually has a magnification power of 10X or 15X. The body tube is the long tube that connects the eyepiece to the objective lenses.
• The Objective lenses and nosepiece: The objective lenses are the lenses that are closest to the specimen and provide the primary magnification. A standard microscope has three or four objective lenses that range from 4X to 100X in magnification power. The nosepiece is a rotating turret that holds the objective lenses and allows you to switch between them.
• The Stage and stage clips: The stage is the flat platform where you place the slide with the specimen. The stage clips are metal clips that hold the slide in place on the stage.
• The Illuminator and condenser: The illuminator is the light source for the microscope, usually a low-voltage bulb. The condenser is a lens system that gathers and focuses the light from the illuminator onto the specimen.
• The Iris diaphragm and aperture: The iris diaphragm is a device that adjusts the amount of light that reaches the specimen by opening and closing a circular hole. The aperture is the hole in the middle of the stage that allows light from the condenser to reach the specimen.
• The Coarse and fine adjustment knobs: The coarse adjustment knob is used to move the stage up and down to bring the specimen into focus. The fine adjustment knob is used to make small adjustments to the focus by moving the stage slightly.
• The Arm and base: The arm is the part that connects the body tube to the base of the microscope. The base is the bottom part that supports the microscope and houses the illuminator.

The following diagram shows the parts of a compound microscope:

Compound microscopes are widely used in various fields of science, medicine, education and industry. They enable us to observe and study the structure and function of microscopic objects that are otherwise invisible to the naked eye. Some of the applications of compound microscopes are:

• Pathology: Pathology is the study of diseases and their causes, effects, diagnosis and treatment. Compound microscopes are essential tools for pathologists to examine tissue samples, blood samples, body fluids and other specimens for signs of infection, inflammation, cancer and other abnormalities.
• Forensics: Forensics is the application of science to law and criminal investigation. Compound microscopes are used by forensic scientists to analyze physical evidence such as hair, fibers, fingerprints, DNA, bullets and drugs.
• Mineralogy: Mineralogy is the study of minerals and their properties, formation and distribution. Compound microscopes are used by mineralogists to identify and classify minerals based on their physical and chemical characteristics.
• Education: Compound microscopes are used by teachers and students in schools and colleges to conduct experiments and demonstrations in various subjects such as biology, chemistry, physics and geology.
• Biotechnology: Biotechnology is the use of living organisms or their products for industrial or medical purposes. Compound microscopes are used by biotechnologists to manipulate and modify biological materials such as DNA, proteins, enzymes and cells.

Compound microscopes have several advantages that make them useful for various purposes. Some of the main advantages are:

• Simplicity and convenience: A compound microscope is relatively easy to use and operate, as it does not require any complex adjustments or alignments. It also comes with its own light source, so it can be used in any environment. Moreover, it is small and portable, which makes it simple to store and transport.
• High magnification and resolution: Because of their multiple lenses, compound microscopes can achieve high magnification levels, up to 1000x or more. This allows them to reveal a great amount of detail in small specimens, such as cells, bacteria, fibers, crystals, etc. They also have a high resolving power, which means they can distinguish between two points that are very close together.
• Versatility and diversity: A compound microscope can be used for a wide range of applications and fields, such as biology, medicine, forensics, geology, chemistry, education, etc. It can also be modified or adapted with different accessories and techniques, such as stains, filters, polarizers, fluorescence, phase contrast, etc., to enhance the contrast or visibility of certain features or structures in the specimens.
• Affordability and availability: A compound microscope is relatively inexpensive compared to other types of microscopes, such as electron microscopes or scanning probe microscopes. It is also widely available and accessible in most laboratories, schools, hospitals, and other institutions. Therefore, it is a cost-effective and practical tool for scientific research and education.

These are some of the main advantages of compound microscopes that make them valuable and popular instruments for microscopy. However, they also have some limitations and disadvantages that should be considered before using them. For example, they have a limited depth of field and field of view, which means they can only focus on a thin layer of the specimen at a time and show a small area of the specimen on the screen. They also suffer from chromatic and spherical aberrations, which are distortions or errors in the image caused by the different wavelengths or angles of light passing through the lenses. These limitations can be overcome or minimized by using higher quality lenses or correcting devices.

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