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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The primary magnification of the optical system is provided by the objective lenses. Objective lenses are, in effect, the heart of the microscope. Some simple microscopes, particularly most student model microscopes are equipped with three objectives having magnification of about 2.5X, 10X, and 40X. These objective lenses are either detached and interchanged by releasing a small catch or may be mounted on a rotating nosepiece. The measure of the size of the cone of light that it a lens can accommodate is called the numerical aperture (NA) of a lens. 


            The measure of the ability to reveal fine detail is called the resolution of a lens. The smallest distance between two points that can still be distinguished is the limit of resolution. When a limit of resolution is small, this means that the lens has a high resolving power. It should be noted that as the numerical aperture increases, the limit of resolution goes up and down and the resolving power of the lens goes up. Also, high numerical apertures are particular only with high magnification lenses. For instance, if the material between the lens and the object being examined is air, then n = 1, and the numerical aperture can be no more than 1. It will actually not exceed at 0.95 NA. This is because the angular aperture must be somewhat less than 180 degree. Therefore, it should be apparent that by placing something with a higher index of refraction between the lens and the object being examined, the numerical aperture and the resolving power could be increases substantially. A drop of mineral oil can be placed between then and the microscope slide to increase the numerical aperture for oil immersion lenses. The index of the oil that is usually used is 1.515 and the maximum numerical aperture obtainable is about 1.4. The limit of resolution with an oil immersion lens using light from the middle of the visible spectrum is about 200nm. The numerical aperture of a high power lens on most student model microscopes is about 0.7 and is usually used without oil. In this case, the theoretical limit of resolution is about 400nm. The limit of resolution, in practice, will be substantially worse than the theoretical limit.


            The distance between the top of the slide and the bottom of the objective lens is called the free working distance. The free working distance of several high power objectives is less than a millimeter, while those low power objectives have a free working distance of about few centimeters. Caution is necessary when using a high power objective. This is because it is quite easy to damage either the lens or the microscope slide by forcing the slide against the lens of the petrographic polarizing microscope while focusing. Most high power objectives are constructed and designed accordingly so that the lower part of the lens can be pushed up into the body of the lens against spring tension to reduce hazards in focusing.


            The objective lens can be focused precisely at only one point in time. However, the other objects found below and above the point of precise focus are considered in reasonably sharp focus. The depth of field is the distance between the lower and upper limits of reasonably sharp focus. The depth of field of lenses that are having low magnification and low numerical aperture are relatively large. While those lenses with high magnification and high numerical aperture usually have very small depth of field.


            Most objective lenses are found with marks of its magnification, numerical aperture, length of the microscope tube from which it is designed for, and the thickness of the cover slip that provides the greatest optical efficiency. The thickness of the cover glass is quite important when high power objective lenses are used. In the case of low power objectives, this is not very particular. Oil immersion objective lenses are commonly marked oil or oel. Lenses that are not designed for the use of oil should never be used with oil.

Wednesday, February 27th, 2008 at 3:40 am
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Click Here For Best Selection Of High Quality Polarizing Microscope