Microscopes

Microscopy

 "The use of microscope is known as microscopy. " The first compound microscope was developed by Zacharhs Janssen, in Holland in 1595. It was simply a tube with lenses at each end and its magnification ranged from 3X to 9X. Two important terms are used in microscopy i.e. magnification and resolving power.

 Magnification

 "Magnification is the increase in the apparent size of an object and it is an important factor in microscopy." Resolving power "Resolving Power or resolution is the measure of the clarity of an image." It is the minimum distance at which two objects can be seen as separate objects, The human naked eye can differentiate between two points, which are at least 0.1 mm apart. 
This is known as the resolution of human eye. If we place two objects 0.05 m art, human eye would not be able to differentiate them as two separate objects. The resolution can be increased with the help of lenses. Light microscopy and electron microscopy Now two types of microscope i.e. light microscope (LM) and electron microscope (EM) are used in microscopy.
 When we see a micrograph on the page of a book, we see some words like “LM 109X” printed along the edge of the micrograph. It tells us that the photomicrograph was taken through a light microscope and that the image has been magnified 109 times. 

 Light Microscope

 A light microscope works by passing visible light through a specimen. It uses two glass lenses. One lens produces an enlarged image of the specimen and the second lens magnifies the image and projects it into the viewer's eye or onto photographic film. A photograph taken through a microscope is called a micrograph. 
 A light microscope can magnify objects only about 1500 times without causing blurriness i.e. its magnification is 1500X.
 Its resolving power is 0.2 micrometer (pm) and 1pm = 1/1000 mm. In other words, the LM cannot resolve (distinguish) objects smaller than 0.2 pm. It is about the size of the smallest bacterium. 
The image of bacterium can be magnified many times, but the light microscope cannot show the details of its internal structure amoebae (left), unicellular algae (right)

 Electron Microscope

  It is the most advanced form of microscope. In EM, the object and the lens is placed in a vacuum chamber and a beam of electrons is passed through he object. Electrons pass through or are reflected from the object and make image. 
lectromagnetic lenses enlarge and focus the image onto a screen or photographic film. The EM has much higher resolving power than the LM. The most modern EM can th objects as small as 0.2 nanometer (nm) and 1 nm = 1/1000,000 mm. It is a ousand-fold Improvement over the LM. Linder special conditions EM can detect lndividual atoms. Cells, organelles and even molecules like DNA and protein are much higher than single atoms. Biologists use two types of electron microscopes.

 Transmission Electron Microscope (TEM)

 It is used to study the details of the internal cell structure. The TEM can magnify objects about 250,000 times. In this microscopy specimen is cut into extremely thin sections. When electron beam is directed through the chamber, the electrons hit the specimen and are transmitted. 
The lenses focus the transmitted electron beam onto a screen or photographic film for used to study the detailed architecture of cell surfaces. It uses an electron beam to scan the surface that has been coated with metal. When beam hits the metal, it is not absorbed or transmitted. Electrons are reflected from the metal .