Lens: Main optical part of any microscope

The main optical part of any microscope is the lens. It consists of a complex, centered lens system that provides a correct, magnified inverse image of the subject.

The front lens of the objective (spherical or hemispherical) that magnifies the image is called the front lens. The lenses lying behind it – corrective ones – correct the image by eliminating imperfections – artifacts (aberrations) created by the front lens.

The focal length of the lens for rays of different wavelengths is different. Therefore, when using non-monochromatic light, the image of the object formed by the lens has colored edges. This phenomenon is known as chromatic aberration; it is eliminated by achromatic and apochromatic lenses. The difference in the optical properties of the central and peripheral parts of a spherical lens causes spherical aberrations; they are eliminated by apochromatic lenses. Currently, to eliminate this drawback, special lenses are used – planachromats and planapochromats. In bacteriological practice, the most widely used lenses are apochromats, achromats and planochromats. Such a subdivision of lenses is carried out according to the nature of the correction of aberrations, i.e. defects in the image of optical systems.

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When using apochromatic lenses, chromatic aberration is almost completely absent, i.e. decomposition of white color into components of the spectrum. Consequently, conditions are created for the most correct transfer of the color of the object. This property is provided due to the complexity of optics (up to 10-12 lenses) and the use of special glasses of different chemical composition.

More common are achromats, in which chromatic aberration has been partially eliminated. These lenses contain up to 6 lenses and produce an image that is sharpest in the center.

When microscopy of colored objects using achromats, a yellowish or greenish background may appear around the image. When microphotographing, it is advisable to use planochromats. Planochromats completely eliminate the curvature of the visual field up to the edges.

Each lens is characterized by the following basic constants: focal length, magnification, resolving power, numerical aperture, effective aperture. The native magnification of the lenses is indicated on the frame (8, 10, 20, 40, 60, 90, 100). On some lenses, instead of magnification, the focal length is indicated in millimeters (16; 8; 5; 1.5 mm, etc.). The shorter the focal length of the objective (the front lens facing the preparation), the greater its magnification. According to the focal length, lenses are divided into strong (F = 5-1.5 mm), medium (F = 5-12 mm) and weak (F = 25-30-50 mm). Resolution of the lens, i.e. the property of depicting the smallest details of the preparation, is characterized by the smallest distance at which two closely spaced points are distinguished. Resolution is determined by the formula:


where E is the resolution of the lens;

λ is the length of the light wave;

A – numerical aperture.

Numerical aperture (“coverage” of the lens) is the product of the refractive index of the medium separating the object from the front lens of the microscope objective by the sine of the aperture angle.

According to the method of use, lenses are dry and immersion, i.e., immersed. Dry lenses are those in which there is air between the front lens and the preparation. These lenses are characterized by low magnification and relatively long focal lengths.

Some lens specifications are engraved on their frame. TO

they include (Fig. 2 and 3):

– increase (4, 10, 40, 100, etc.),

– aperture (0.12; 0.30; 0.65; 1.25),

– length of the tube (160, etc.),

– thickness of the cover glass (0.17),

– type of immersion (MI – oil immersion – black rim, VI – water immersion – white rim)

When microscopy with strong lenses, the focal length of which is small, it is necessary to create a homogeneous optical medium between the front lens of the objective and the preparation glass. This is achieved by immersing the lens in a drop of cedar oil on the preparation. Cedar oil has a refractive index of n = 1.510, close to that of glass. In this regard, the light beam leaving the glass is not scattered and, without changing its direction, enters the lens, providing good illumination. Peach oil (n = 1.471-1.498), a mixture of castor and dill oils (n = 1.474-1.498), etc. can be used as substitutes for cedar oil.

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