Lapping is an operation that can be done by hand or machine where two surfaces are rubbed together with an abrasive in
the middle to try to get the surfaces as flat and as smooth as possible.
This can take two forms. The first type of lapping (traditionally called grinding), typically involves rubbing a brittle
material such as glass against a surface such as iron or glass itself (also known as the "lap" or grinding tool) with
an abrasive such as aluminum oxide, emery, silicon carbide, diamond, etc., in between them. This produces microscopic
conchoidal fractures as the abrasive rolls about between the two surfaces and removes material from both.
The other form of lapping involves a softer material for the lap, which is "charged" with the abrasive. The lap is then
used to cut a harder material?the workpiece. The abrasive embeds within the softer material which holds it and permits it
to score across and cut the harder material. Taken to the finer limit, this will produce a polished surface such as with
a polishing cloth on an automobile, or a polishing cloth or polishing pitch upon glass or steel.
Taken to the ultimate limit, with the aid of accurate interferometry and specialized polishing machines or skilled hand
polishing, lensmakers can produce surfaces that are flat to better than 30 nanometers. This is one twentieth of the
wavelength of light from the commonly used 632.8 nm helium neon laser light source. Surfaces this flat can be molecularly
bonded (optically contacted) by bringing them together under the right conditions.