网范文：“Microscopy in Biology” 在这篇生物范文中，讲述了显微镜对人们观察细胞的影响。在古代,由于没有显微镜,只有对有机体的宏观结构进行探讨。近代以来,由于显微镜的发明,微观维度的世界被发现。原生动物,由许多细胞分组和分化而成，组织和器官的形成也一样。显微技术的发展和改进促使人们进一步了解细胞结构。观察生物结构是很困难的，因为细胞非常小,而且对可见光透明。除了一些色素存在于植物细胞，英语论文，大部分处于透明状态。他们吸收特定波长的光，活细胞的低光吸收主要是由于其高含水量。为了克服这个困难,有选择地对不同的细胞成分染色，英语论文范文，产生对比光吸收。)
In olden times, due to non availability of any kind of microscope, only macroscopic structures of an organism were studied. Later, owing to the invention of magnifying lenses, the world of microscopic dimensions was discovered.
It was found that a single cell can constitute an entire organism as in Protozoa, or it can be of many cells that are grouped and differentiated into tissues and organs to form a multicellular organism. The development and refinement of microscopic techniques made it possible to gain further knowledge of cellular structure.
Observation of biological structures is difficult since cells are very small and are transparent to visible light. The majority of cell components are transparent, except for some pigments present in plant cells. They absorb light at certain wavelengths (coloured substances). The low light absorption of the living cell is caused largely by its high water content. The cell components show little contrast even after drying. To overcome this difficulty, we use dyes that selectively stain different cell components to produce contrast by light absorption.
Resolving Power:
The purpose of any microscope is to make visible an object that normally can not be seen, or can not be seen clearly, by the naked eye. The property of a microscope is their ability to clearly distinguish separate parts of an image. This property of microscope is termed its resolving power.
The resolving power of a microscope is the capacity to show distinct images of points which are very close together or the smallest separation at which we can distinguish two objects rather than one.
Thus, for example, if two parts of an image are 0.01 µm apart, they can be resolved as separate entities by an electron microscope, which has a resolving power of 0.5 nm (0.0005 µ.m), but not by a light microscope, which has a resolving power of 0. 2 µm.
The resolving power of a light microscope is approximately equal to one half the wavelength of light used to illuminate the object. The resolving power depends upon the wavelength (lambda, λ) and numerical aperture (NA) of the objective lens. Limit of resolution is defined as the minimum distance between two points that allows for their discrimination as two separate points.
Limit of resolution (r) = 0. 61 λ / NA
NA is the numerical aperture and it is equal to n x sin α. Here, n is the refractive index of the medium and sin α is the sine of the semi angle of aperture.
The limit of resolution is inversely related to the resolving power, i.e., the higher the resolving power, the smaller the limit of resolution.
Since sin α can not exceed 1, and the refractive index of most optical material does not exceed 1.6, the maximal NA of lenses, using oil immersion, is about 1.4. With these parameters it is easy to calculate the limit of resolution of the light microscope that can not exceed 170 nm (0.17 µ.m) using monochromatic light of λ= 400 nm (violet). With white light, the resolving power is about 250 nm (0.25 µm). Since here the NA is limited, it is evident that the only way to increase the resolving power is to use shorter wavelengths.)
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