A compound microscope is a light microscope that uses a combination of lenses to magnify small specimens. It has an objective lens close to the specimen that forms a real image and an eyepiece lens (ocular) that magnifies this image for viewing.
Explanation
In a compound microscope, light from an illuminator or mirror passes through the specimen on the stage and is focused by the objective lens. The objective produces an enlarged, inverted image within the body tube. The eyepiece lens further enlarges this intermediate image, giving a total magnification equal to the product of the two lenses. Most microscopes have several interchangeable objective lenses mounted on a revolving nosepiece, typically 4×, 10×, 40× and 100× oil immersion, along with a 10× or 15× eyepiece. Fine and coarse focus knobs adjust the distance between the objective and the specimen to obtain a sharp image. A condenser and diaphragm control the amount and focus of light entering the objective.
The resolving power of a compound microscope depends on the numerical aperture of the objective and the wavelength of light. Using immersion oil with the highest‑power objective reduces light refraction and increases resolution. Bright‑field microscopes produce images based on differences in light absorption; variations include phase‑contrast and dark‑field microscopes that enhance contrast in unstained specimens. Proper cleaning and handling of lenses and regular calibration are necessary to maintain image quality.
Applications and examples
Compound microscopes are essential tools in biology and medicine. They are used to examine bacteria, protozoa, algae, blood smears and tissue sections. Students use them to learn cell structure and observe mitosis. In clinical laboratories they assist in diagnosing infections, counting cells and identifying parasites. Advances such as LED illumination, digital cameras and trinocular heads have modernised the traditional design. Despite the availability of electron microscopes, the compound light microscope remains the workhorse for routine microbiology and pathology.
Proper use of a compound microscope requires careful adjustment of illumination, focusing and lens selection. Mastery of these skills allows observation of cellular details that are invisible to the naked eye.
Related Terms: objective lens, eyepiece, bright-field, magnification, resolution