The point(s) from which light rays diverge as they enter a lens or mirror. See: virtual object.

The point(s) to which light rays converge as they enter a lens. The rays pass through each object point. See: real object.

Do not call an authoritative or ‘book’ value of a physical quantity a theoretical value, as in: 'We compared our experimentally determined value of index of refraction with the theoretical value [..]

One hears some folks with superficial minds say 'Einstein showed that everything is relative.' In fact, special relativity shows that only certain measurable things are relative, but in a pr [..]

Many books emphasize that the mole is 'just a number,' a measure of the number of particles in a collection. They say that one can have a mole of any kind of particles, baseballs, atoms, sta [..]

Be careful that the reader does not confuse this with the colloquial usage: 'One factor in the success of this experiment was…'

The uncertainty in a measured quantity is due to inherent variations in the measurement process itself. The uncertainty in a result is due to the combined and accumulated effects of these measurement [..]

The point(s) from which light rays converge as they emerge from a lens or mirror. The rays do not actually pass through each image point, but diverge from it. See: real image.

The size of the external force required to keep a body at rest in its frame of reference.

Labels which distinguish one type of measurable quantity from other types. Length, mass and time are distinctly different physical quantities, and therefore have different unit names, meters, kilogram [..]