intensity

Here intensity becomes something there can be more or less of, so it takes a plural when levels are compared: trees that grow best under high light intensities. Adjectives like high attach naturally, and the sense bridges everyday talk and the laboratory. In criticism it drifts back toward the quality of being intense, as when a portrait's eyes are "full of intensity and life."

A seismologist's word for what an earthquake does where you stand, not for the earthquake itself. A single quake has one magnitude but many intensities, because shaking is strongest near the epicenter and fades with distance. Damage surveys therefore map it town by town on descriptive scales such as the Modified Mercalli. When a report calls a quake strong, check which of the two measures it means.

Physics turns the word into a strict quantity, power spread over area, measured in watts per square meter and applied to sound as readily as to light. Because it is a ratio, spreading a fixed power over twice the area halves the intensity. Take care when reading optics, where the optics cover term can also mean power per solid angle rather than per area.

A word optics uses loosely, which is why careful writers replace it with the specific quantity they mean. Radiant intensity is power per solid angle, luminous intensity is measured in candela, and irradiance is power per area, the meaning the physicist's energy flux fixes. When the bare word appears, the surrounding context has to settle which one is meant.

Astronomers mean something narrower again, with solid angle built in, so the units are watts per square meter per steradian. That extra term explains a useful fact: the intensity of a resolved source, such as a disk seen across space, does not fade with distance, even though the total light received does. Readers moving between astronomy and optics texts should check which quantity the word names.