What term refers to how bright light appears to an observer?

The term that refers to how bright light appears to an observer is intensity. Intensity is a measure of the power of light per unit area, and it directly relates to the perceived brightness of that light. When light reaches an observer's eye, its intensity determines how bright or dim the light appears. This perception can be influenced by various factors such as the observer's distance from the light source, the surrounding environment, and the characteristics of the observer's vision.

Brightness, while related to the observer's perception, does not convey a specific measurement. Clarity typically refers to the quality of being clear or understandable and is not directly related to the perception of light. Luminosity, on the other hand, is a measure of the total amount of light emitted by a source and does not account for the distance from the observer or the area over which that light is spread. Thus, these terms are contextually different from intensity, which specifically addresses the perception of brightness as experienced by an observer.