Ball lens

As a lens, a transparent sphere of any material with refractive index (n) greater than air (n > 1.00) bends parallel rays of light to a focal point.

For most glassy materials the focal point is only slightly beyond the surface of the ball, on the side opposite to where the rays entered.

The first lenses were likely spherical or cylindrical glass containers filled with water, which people noticed had the ability to focus light.

The rays travel in straight lines within the lens, and then are bent again when they exit, converging to a focal point which is typically just outside the ball.

Due to the optical invariant, this allows light from a collimated beam to be focused to smaller diameters than could be achieved with other spherical lenses.

Similarly, a point source of light placed at the focal point will produce a collimated beam emanating from the opposite side of the lens, and the lens's large ratio of diameter to focal length (large numerical aperture) allows more light to be captured than would be possible with other spherical lenses.

In addition, ball lenses are omnidirectional, which eases alignment of optical couplings over other types of lens because all that is necessary is to keep everything centered.

[4] In 1677, Antonie van Leeuwenhoek used a small ball lens to create a single-lens microscope with 300× magnification, allowing the first observation of spermatozoa.

[9] For materials with refractive index greater than 2, objects at infinity form an image inside the sphere.

Many of those few are either too brittle, too soft, too hard, or too expensive for practical lens making (columbite, rutile, tantalite, tausonite).