Polarizing Microscope (A Type of microscope )

Polarizing Microscope 

A polarizer, or a polarizing filter, may be imagined as a sieve which filters ordinary light waves (which vibrate in all directions) and allows only light waves of one orientation (e.g., north-south) to pass through the filter. Thus polarized light vibrates only in one plane. Some crystalline substances are able to filter out all other planes of vibration from ordinary light except the one which lies on the optical axis of the substance. One such substance is the Icelandic spar. 

This can be cut and mounted to form structures known as Nicol prisms.In a polarizing microscope, a polarizing filter, such as Nicol prism, is placed between the light source and the specimen.

 The second polarizer, called analyser, is placed above the specimen between the the objective and the eyepiece. One of the polarizers is then rotated until the two are at right angle to each other. This will be seen as absence of light through the eyepiece bacause the two polarized light waves will cancel each other (e.g. north-South and east-West waves). However, certain objects have a property called birefringence, which means that they rotate or polarize light. The objects that can polarize light can be seen with this system and will appear bright against a dark background.

In recent years, other polaroid material has replaced Icelandic spar. Although it does not give as dark a background as the latter, it is just adequate for most purposes.

The polarizing microscope is useful in a clinical laboratory for the study of synovial fluid and urinary sediment, and in some histological work; and sometimes in forensic medicine. It is also used for the detection of the presence of silica, asbestos, mica and talc in granulomas, and in the identification of hooklets of certain parasites. This technique is more commonly used in geology for particle analysis.

Projection Microscopy

This is used for the projection of enlarged images of tissue sections on a microscope glass slide. The slide is projected onto a suitable screen for visual aid or demonstration.

There are two types of projection microscopes:

1. The micro projector, suitable for a limited audience. It has reduced light intensity and therefore, poor resolution.

2. The other large model gives a large, bright image over long distances. However, the slides tend to fade with prolonged projection.