Compound Microscope:Definition,Working & 5 Important Uses

What are compound microscopes?

A compound microscope is also known as a biological microscope as it is mainly used for observing biological tissues, blood cells, bacteria, cheek cells, algae, etc. A compound microscope is designed in a way that one of the lenses gathers light from the specimen slide and focuses a real image of the same internally and the other lens further enlarges that real image resulting in the formation of an inverted vertical image. A microscope slide is used for placing the sample which is then covered with a coverslip.

compound microscope
An ultra-fine measurement Microscope with slide in graticule and vernier rotating table Image source: (types of microscope)Les Chatfield from Brighton, England, Fine rotative table Microscope 5 (12996283235)CC BY 2.0

What is the magnification of compound microscopes?

Compound microscopes can provide high magnifications that can be applied to a number of purposes. The most popular objective of compound microscopes has magnifications ranging from 5x to 100x and numerical apertures from 0.14 to 0.7. The magnification of a microscope eyepiece can be varied according to the desired magnification.

Eyepiece magnification generally has 5x, 10x, 15x, and 20x orders. With the decrease in objective magnification its resolution increases and vice-versa. Hence, for large magnifications, we might not able to differentiate between two points properly. In order to increase the resolution of the microscopic image, the numerical aperture has to be increased.

How does a compound microscope work?

In a typical compound microscope, the sample is aligned on the microscope stage first. The sample is then illuminated (either from underneath or overhead) depending on the light requirements. An erect real image is formed inside the microscope tube when the light rays from the sample pass through the microscope objective lens.

The real image is produced at the focal point of the microscope eyepiece lens and is known as the intermediate image. Finally, an inverted magnified image of the sample is produced when the light rays coming out of the focused point pass through the eyepiece lens.

In certain microscopic designs, one may observe the use of a CCD (charge-coupled device) in the microscopic tube. This CCD replaces the functioning of the eyepiece lens. For such a CCD-microscope design, the final image is displayed on a monitor. The CCD has to be placed on the plane of the intermediate image for this process.

Ray diagram of a compound microscope. Image source: Fountains of Bryn MawrMicroscope compound diagramCC BY-SA 3.0

What are the uses of compound microscopes?

Apart from observing biological samples, a compound microscope is used for a variety of purposes.

  • Sometimes compound microscopes are used along with special phase contrast objective lenses and a phase condenser or a phase slide for bringing forward the contrast of a sample specimen without straining the quality of the image of the sample specimen. These phase contrast microscopes are used for examining bacterial cells and blood cells.
  • Compound optical microscopes are at times modified by adding a polarizer and an analyzer near the light source. The polarizer is responsible for allowing only some specific light wave to pass into the microscope. The analyzer is responsible for determining the intensity of light that should fall on the sample as well as the direction of light. The polarizer further directs different wavelengths of light on the same plane. These microscopes are used especially by geologists, chemists, petrologists, and the pharmaceutical industry for examining rocks, minerals, chemicals, and medicines. The polarizing microscope is a modified form of a compound microscope with a polarizer and an analyzer placed on the light source.
  • Compound optical microscopes in which the light source placed in such a way that the light reflects off the sample instead of transmitting. Metallurgical microscopes are specially designed for observing samples that do not permit light rays to pass through them. These microscopes are used for examining micron level cracks in metals, extremely fine layers of coating like paint and lamination, and for grain sizing. Metallurgical microscopes are mainly used in the automobile industry, aerospace industry, ceramic, and polymer industries.
  • A fluorescence microscope is a compound microscope variant that is designed with several different light sources of producing various wavelengths of light to fluoresce a given sample. These microscopic designs are efficient for analyzing biological samples.
  • Differential interference contrast (DIC) microscopes are also a modification of compound optical microscopes.
A metallurgical microscope. Image source; (types of microscope) Jeff Keyzer from Austin, TX, USA, American Optical 569 Stereo Star Zoom MicroscopeCC BY-SA 2.0

How to use a compound microscope?

Optical microscope components. Eyepiece (ocular lens)(1), Objective turret, revolver, or revolving nose piece(2), Objective lenses(3), Coarse adjustment(4), Fine adjustment(5), Mechanical Stage(6), Light source(7), Diaphragm and condenser(8)image source: GcG(jawp), Optical microscope nikon alphaphot, marked as public domain, more details on Wikimedia Commons

Step 1: Place the sample on the microscope stage.

Step 2: The sample has to be illuminated with appropriate light intensity.

Step 3: The required objective magnification should be selected.

Step 4: The coarse and fine adjustment focus knobs should be used to bring the specimen sample to proper focus.

Step 5: Now, see through the eyepiece to observe the specimen. / If the microscope has a CCD, then the sample would be shown on the monitor.

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Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers. Let's connect through

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