Construction of a Compound Microscope
Back to Top
A compound microscope consists of the following parts:
The objective lens of a compound microscope is a convex lens of very short focal length (f
o) that is f
o < 1cm. The object to be seen is kept very close to the objective lens.
The eye piece of a compound microscope is also a convex lens of short focal length f
e. But f
e > f
o.
The 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.
Working
Back to Top
The 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 (I
1) of the object. The image I
1 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 (F
e). 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.
Magnifying Power of a Compound Microscope
Back to Top
The magnifying power of a compound microscope is defined as the ratio of the size of the final image (I
2) as seen through the microscope to the size of the object as seen with a naked eye.
Image Formation in a Compound Microscope
= m
objective x m
eyepiece
= mo x me
Where m
objective (m
o) and m
eyepiece (m
e) are the magnification produced by the objective and eyepiece respectively.
m = mo x me
Eye piece is nothing but a simple microscope
The lens formula is
But distance between the object and the lens is -u.
Multiply equation (2) by V
m = m
o x m
e
