Lens (geometry) in the context of Convex set

A nummulite is a large lenticular fossil, characterised by its numerous coils, subdivided by septa into chambers. They are the shells of the fossil and present-day marine protozoan Nummulites, a type of foraminiferan. Nummulites commonly vary in diameter from 13 to 50 mm (0.51 to 1.97 in) and are common in Eocene to Miocene marine rocks, particularly around southwest Asia and the Mediterranean in the area that once constituted the Tethys Ocean, such as Eocene limestones from Egypt or from Pakistan. Fossils up to six inches wide are found in the Middle Eocene rocks of Turkey. They are valuable as index fossils.

The ancient Egyptians used nummulite shells as coins and the pyramids were constructed using limestone that contained nummulites. It is not surprising then that the name Nummulites is a diminutive form of the Latin nummulus 'little coin', a reference to their shape.

The lentil (Vicia lens or Lens culinaris) is an annual legume grown for its lens-shaped edible seeds or pulses, also called lentils. It is about 40 cm (16 in) tall, and the seeds grow in pods, usually with two seeds in each.

Lentil seeds are used around the world for culinary purposes, most commonly in stews or soups. In cuisines of the Indian subcontinent, where lentils are a staple, split lentils (often with their hulls removed) known as dal are often cooked into a thick curry that is usually eaten with rice or roti.

In plane geometry, a lune (from Latin luna 'moon') is the concave-convex region bounded by two circular arcs. It has one boundary portion for which the connecting segment of any two nearby points moves outside the region and another boundary portion for which the connecting segment of any two nearby points lies entirely inside the region. A convex-convex region is termed a lens.

Formally, a lune is the relative complement of one disk in another (where they intersect but neither is a subset of the other). Alternatively, if ${\displaystyle A}$ and ${\displaystyle B}$ are disks, then ${\displaystyle A\smallsetminus A\cap B}$ is a lune.

The vesica piscis is a type of lens, a mathematical shape formed by the intersection of two disks with the same radius, intersecting in such a way that the center of each disk lies on the perimeter of the other. In Latin, "vesica piscis" literally means "bladder of a fish", reflecting the shape's resemblance to the conjoined dual air bladders (swim bladder) found in most fish. In Italian, the shape's name is mandorla ("almond"). A similar shape in three dimensions is the lemon.

This figure appears in the first proposition of Euclid's Elements, where it forms the first step in constructing an equilateral triangle using a compass and straightedge. The triangle has as its vertices the two disk centers and one of the two sharp corners of the vesica piscis.