meniscus

Comparison of the appearance of the meniscus in water and in mercury. When a liquid is placed in a thin tube, surface tension causes its surface to curve. A liquid, such as water, that wets the walls of the container has a concave meniscus, but a liquid that does not wet the walls, such as mercury, has a convex meniscus.

In physics, the curved shape of the surface of a liquid in a thin tube, caused by the cohesive effects of surface tension (capillary action). When the walls of the container are made wet by the liquid, the meniscus is concave, but with highly viscous liquids (such as mercury) the meniscus is convex. Also, a meniscus lens is a concavo-convex or convexo-concave lens.