Sangeetha Pulapaka

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.

 A Simple microscope is a microscope that uses only one lens for magnification. It is the original design of the light microscope. Van Leeuwenhoek’s microscopes consisted of a small single converging lens mounted on a brass plate with a screw mechanism to hold the sample or specimen to be inspected. Even though it is now considered prehistoric, the use of a single, convex lens for viewing is still found in simple magnification devices, such as magnifying glass and the loupe. People who generally use microscopes are Doctors and Scientists.

The magnifying power or angular magnification of a microscope may be defined as the ratio of the angle subtended at the eye by the image formed at the distance of the distinct vision to the angle subtended by the object when placed at the distance of the distinct vision.

The ray diagram shows that the image of the object AB is formed at A_{1}B_{1}. A_{1}B_{1} is formed at the least distance of distinct vision.


The figure shows that the angle A1OB1 subtended at the eye by the object in the position A1B1is greater than the angle AOB subtended by it in the position AB. From this it is clear that the eye estimates the angle subtended by an object on it and not the linear size of the object.

Compound means a microscope that has more than one lens and microscope, if divided into two different words as it is, "micro" means small and "scope" means view. Therefore in a whole the meaning is a device that would facilitate the viewing of small organisms with the help of multiple lenses. Simple microscopes and compound microscope differ in the sense that the former uses a single lens and the latter uses more than one. A compound microscope can be used for several purposes ranging from medical research to a day at the beach as your urge to know can only be inhibited by your imagination.

 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 (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.