Compound Microscope

A compound microscope is an optical instrument which is used to magnify very small objects like blood cells, bacteria which otherwise cannot be seen with the naked eye.
A Laboratory Model of Compound Microscope
The essential parts of a compound microscope are two convex lenses of short focal length. These lenses are referred to as:
the objective lens or objective
the eye piece or lens
Construction of a Compound MicroscopeA compound microscope consists of the following parts:
Objective lensThe objective lens of a compound microscope is a convex lens of very short focal length (fo) that is fo <> fo.
Microscope tubeThe objective lens and the eyepiece are mounted coaxially (having a common axis) at the ends of two brass tubes which can be made to slide into each other so that the distance between the two lenses can be adjusted.
WorkingThe ray diagram given below gives the principle of a compound microscope. The object is mounted on the stand below the microscope tube. The objective lens forms a real, inverted and magnified image (I1) of the object. The image I1 acts as an object for the eye piece. The position of the eyepiece is so adjusted that the image lies within the focus of the eyepiece (Fe). The eyepiece acts like a magnifying glass and forms a virtual erect and magnified image of the object.
Image Formation in a Compound Microscope
The object (O) is placed just outside Fo, the principal focus of the objective lens.
Fe is the principal focus of the eye lens.
A real, inverted magnified image I1 is formed. The magnified image I1 acts as an object for the eye lens.
The final image I2 is virtual and is magnified still further. It is inverted compared with the object. I2 may appear 1000 times larger than the object.