What term refers to the inverse of the distance from the observation point to the source?

The term that refers to the inverse of the distance from the observation point to the source is vergence. Vergence is a measure of how much light rays are converging or diverging as they travel through space. It is typically expressed in diopters, with one diopter corresponding to the converging or diverging effect that a lens produces at a distance of one meter.

In practical terms, when light rays originate from a point source and move toward an observer, their vergence decreases as the source moves further away, resulting in decreased visual clarity. Conversely, as the source gets closer, the vergence increases as the rays converge more steeply. This concept is vital in optics and vision science, particularly when considering how lenses and optical systems manipulate light to focus images on the retina.

Other terms, like divergence and convergence, relate to how light rays behave but do not directly represent the mathematical concept of the inverse of distance in the context of optical vergence. Refraction, on the other hand, specifically refers to the bending of light as it passes through different media, rather than the relationship between distance and direction of light rays.