Compound Microscope: Principle, Construction, and Magnification
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Principle of Compound Microscope
A compound microscope operates on the principle that when a small object is positioned just beyond the objective lens's focal point, it produces a virtual, inverted, and highly magnified image. This final image is formed at the least distance of distinct vision (25 cm) from the eye, which is positioned close to the eyepiece.
Construction of a Compound Microscope
A compound microscope is constructed with two convex lenses: an objective lens (O) with a small aperture and an eyepiece (E) with a larger aperture. The lens positioned closer to the object is termed the objective lens, while the lens closer to the observer's eye is called the eyepiece. Both the objective and eyepiece lenses have short focal lengths and are mounted at the free ends of two sliding tubes, allowing for adjustable distance between them. While both lenses have short focal lengths, the objective lens (O) typically has a slightly shorter focal length than the eyepiece (E).
The eyepiece is designed with a larger focal length and aperture to capture more light rays from the magnified object, thereby forming a brighter image.
Working Principle of the Compound Microscope
The operational principle of a compound microscope is best understood through its ray diagram. A small object (AB) intended for magnification is placed in front of the objective lens, just beyond its principal focal point (fo'). The objective lens (O) then forms a real, inverted, and enlarged intermediate image (A'B') of the object.
This intermediate image (A'B') subsequently acts as the object for the eyepiece (E). The eyepiece's position is adjusted so that A'B' falls between its optical center (C2) and its focal point (fe'). The eyepiece then forms the final virtual, inverted, and highly magnified image (A''B''). This final image (A''B'') is observed by the eye, positioned close to the eyepiece, after being adjusted to appear at the least distance of distinct vision (25 cm) from the eye.
Ray Diagram of a Compound Microscope
Magnification of a Compound Microscope
The total magnification (m) of a compound microscope is calculated using the following formula:
m = 
Where:
- D = Least distance of distinct vision (25 cm)
- L = Length of the microscope tube
- fo = Focal length of the objective lens
- fe = Focal length of the eyepiece lens