You can measure the thickness with an optical microscope (look at a crossection) or an ultrasonic probe.

A definition on top of my head, it's the number of frequencies allowed to be carried by the Optical Fiber Cable, It depends on the Ko, thickness and Numerical Aperture of the Fiber Cable.

Oil on top of water will settle as a thin layer, and the thickness of the layer will, due to an optical phenomena called interference, cause the thin layer to shimmer in different colors.

In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. Lenses whose thickness is not negligible are sometimes called thick lenses.