Feb 16 2008
Introduction
Type of Lenses Used in Optical Microscopes
Focusing Light in Optical Microscopes
Contrast in Optical Microscopes
Introduction
The idea of an an optical microscope or a light microscope is to magnify images which are far too small to be seen by the naked eye. Three objectives must be accomplished by an optical microscope:
- Generating a magnified image of the object or specimen being examined
- Separating the details in the magnified image
- Making the details visible
Type of Lenses Used in Optical Microscopes
In an optical microscope, light must gathered from a small area of a well illuminated thin specimen in close distance. As a result, an optical microscope employs small and spherical objective lenses. This allows for a much shorter focal length and hence a much shorter distance is needed within the tube of the microscope to bring the image of the object into focus. A second lens normally referred to as the eyepiece or an ocular lens is used to magnify the image as the image is brought to the eye.
The anatomy of an optical microscope normally consists of interchangeable objective lenses with a fixed eyepieces or ocular lens. Changes in the magnification of a microscope are achieved by varying the shape of the objective lens. A relatively flat objective lens is used in low magnification applications. On the other hand, high magnification can be achieved through the use of a more spherical objective lens.
Focusing Light in an Optical Microscope
In order to focus light in an optical microscope, a condenser is used. A condenser can be considered as a system of lenses that focuses or condenses the light source onto the specimen as a bright and tiny spot. The size of this light spot is approximately the same as the objective lens used.
Contrast in Optical Microscopes
Unfortunately, light microscope does have a drawback. The image of specimens observed under a microscope does not have much of a contrast. This is especially the case for living things such as cells.
To improve contrast, specimens are treated with colored dyes or pigments which bind to certain structures within the specimen. Various types of microscope have been developed to increase the contrast in the specimen.
The difference between various types of microscope lies in the illumination systems used and the type of light that passes through the specimen. For example, a special condenser in a darkfield microscope blocks out most of the bright light and oblique light is used to illuminate the specimen. This type of microscope provides a totally dark background and the contrast of the image are enhanced which brings out the fine details.
Source: AZoOptics