Meniscus(redirected from meniscate)
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the crescent-shaped inner and outer intraarticular cartilages in the knee joint.
The menisci increase the congruence of the joint surfaces, making diversity of movement possible and softening the effect of impact. The menisci may be injured by forced movement connected with overextension of the joint (most often in athletes). When the menisci are detached, there is a sudden block of the joint; sharp pains result from strangulation of the detached parts and the impossibility of movement in the joint. Proper treatment (conservative or, when this proves unsuccessful, surgical) completely restores joint function. In other joints (for example, mandibular and radiocarpal) intraarticular cartilages with analagous functions are called disks.
in optics, a concavo-convex or convexo-concave lens bounded by two spherical surfaces; such a lens is one of the most widely used types of lenses. A meniscus whose thickness is greater at the center than at the edges (positive meniscus) is a converging lens, and a meniscus whose thickness is greater at the edges than at the center (negative meniscus) is a diverging lens. Menisci are used in various optical systems, for example, in eyeglasses, in the objectives of motion-picture cameras and still cameras, and as attachment lenses that change the focal lengths of objectives.
Meniscus systems have gained widespread use in applications that require a higher degree of correspondence between the optical image and the object, for example, in astronomy. In these systems, small distortions of the image introduced by the meniscus (aberrations) are compensated by aberrations introduced by other elements of the system. The result is that it is possible to obtain practically undistorted images. The inherent aberrations of the individual menisci may be reduced by the use of diaphragms (for example, in camera objectives, up to a relative aperture of 1:11).
the curved free surface of a liquid near the contact between the liquid and the surface of a solid. For example, a meniscus is formed at the walls of vessels and in the channels and pores of spongy materials impregnated with liquids. A meniscus has a spherical shape in thin (capillary) tubes and a cylindrical shape in a sufficiently narrow gap between flat plates. The curvature of the meniscus is determined by the relationship between the forces of molecular interaction at the triple point of the three phases—solid, liquid, and gas (or vapor). A liquid that wets a given surface forms a concave meniscus, whereas a liquid that does not wet the surface forms a convex meniscus. In the first case, the mutual attraction between the molecules of the liquid (cohesion) is weaker than the attraction of the molecules of the liquid by the molecules of the surface of the solid (adhesion). Conversely, in the second case, the forces of cohesion exceed the forces of adhesion. The vapor pressure over a concave meniscus is less than, and that over a convex meniscus is more than, the vapor pressure over a plane liquid surface. This explains such phenomena as capillary condensation, capillary absorption of a liquid in porous and fibrous materials, and the rising and falling of liquids in thin pipes.