A microscope is an instrument used to produce enlarged images of small objects. The most common kind of microscope is an optical microscope, which uses lenses to form images from visible light. The optical microscope has been a standard tool in life science as well as material science for more than one and a half centuries now. The optical microscopes are of two main types:
a) Simple Microscope b) Compound Microscope
Figure 1. Parts of a simple microscope
The simple microscope is generally considered to be the first microscope. It was created in the 17th century by Antony van Leeuwenhoek, who combined a convex lens with a holder for specimens, magnifying between 200 and 300 times.
The structure of a simple microscope is very simple. It has a body, a stage made of thick glass for placing slides, two clips to fix the slides, a handle, an eyepiece placed with the handle and a mirror. There is an adjustment knob by which the eye piece may be moved upward and downward. The entire body stands on the base or foot.
How to handle the simple microscope?
At first the slide is to fix on the stage with the help of clips. The light should be focused on the slide by moving the mirror. Then keeping the eye in the eye piece the adjustment knob should move in such a way that the eyepiece comes to a position from where the object on the stage can be seen most clearly. At this position, keeping the eye in the eyepiece, the specimen is to be observed carefully, or if required be dissected with the help of forceps or needle whichever is convenient.
Principle: 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.
Figure 2. Working of a simple microscope When an object is placed between the principle focus and the optical centre of a convex lens a virtual, erect and magnified image is formed. The ray diagram is given here explains how an object looks enlarged when viewed through a bi-convex lens. Observe that the object and the image are on the same side of the lens. Image is formed at the least distance of distinct vision from the eye i.e., the distance from the object is approximately equals to 25 cm.
Magnification of Simple Microscope
The magnifying power of a simple microscope is given by:
m = 1 + D/f
D = least distance of distinct vision
F = focal length of the convex lens
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.
Limitations of using a light microscope: Visible light restricts the amount of resolution achieved with a light microscope. Magnification may range from 500x to 1500x. In general, light microscopes require specimens to be thin, small and transparent for optimum viewing. The diffraction limits the resolution to approximately 0.2 µm.
Uses of a simple microscope:
2. A light microscope is also referred to as a?
a) Electron microscope
b) Compound microscope
c) Scanning problem microscope
3. On the microscope stage, what is used to hold the glass slide in place and prevent it from moving?
a) Stage clip
c) Fine adjustment knob
4. Ocular lens
a) Is used to regulate the amount of light on the specimen
b) You look through to see the specimen
c) Projects light upwards through the diaphragm, the specimen, and the lenses
5. What happens when light passes through a condenser lens?
6. What does the objective lens do?
7. What is the maximum magnification of a light microscope?
8. What is the maximum resolution of a light microscope?
9. What are the advantages of a light microscope?