A microscope is an optical instrument which is used to see highly magnified images of tiny objects such as bacteria, cells , viruses and protozoans etc. because these tiny objects cannot be seen by naked eyes. In this chapter we shall study two types of microscopes:
1. Simple microscope
2. Compound microscope
A simple microscope is also called magnifying glass. It is actually a convex lens of small focal length, which is used for seeing the magnified images of small objects.
Principle of Simple Microscope
A simple microscope works on the principle that when a tiny object is placed within its focus, a virtual, erect and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the lens.
Working of Simple Microscope
The ray diagram to show the working of simple microscope is shown in figure. A small object AB which is to be magnified is placed between the principal focus F’ and optical centre C of the convex lens. Now, a ray of light AO parallel to principal axis which is coming from the point A of the object passes through the focus F along the straight line OX after getting refracted by the convex lens. A second ray of light AC coming from the point A of the object passes through the optical centre C of the convex lens along the straight line CY. As is clear from the figure that the two rays i.e. OX and CY are diverging rays so these rays can intersect each other only at point A’ when produced backward. Now, on drawing A’B’ perpendicular from point A’ to the principal axis, we get the image A’B’ of the object which is virtual, erect and magnified.
Uses of Simple Microscope
Following are the important uses of simple microscope:
- The simple microscope is commonly used by watch makers to see the magnified view of small parts of a watch.
- It is also used by the jewelers to see the magnified view of the fine parts of jewellery.
- Simple microscope is used to see the enlarged image of letters of a book, textures of fibers or threads of a cloth.
- Simple microscope is used to see the magnified view of different particles of different types of soils.
- It is used by palmists to see enlarged view of the lines of our hand.
- Simple microscope is used by skin specialists to find out various diseases of skin.
- It is also used to see the details of stamp and engravings.
Magnification of Simple Microscope
The magnifying power of a simple microscope is given by:
m = 1 +
Where, D = least distance of distinct vision
F = focal length of the convex lens
It should be noted that the focal length of the convex lens should be small because smaller the focal length of the lens, greater will be its magnifying power. Also the maximum magnification of a simple microscope is about 10, which means that the object will appear 10 times larger by using the simple microscope of maximum magnification.
Test Your Understanding and Answer These Questions:
- Define microscope.
- Name types of microscope.
- What is a simple microscope?
- Give uses of simple microscope.
- Which type of lens is used as a simple microscope?
- Explain the principle, construction and working of simple microscope with the help of a ray diagram.
- Explain magnification of a simple microscope. How it can be increased?
- The focal length of lens used in a simple microscope should be small. Why?