principal focus

noun

, Optics.

1. focal point ( def 1 ).

principal focus

noun

1. See focal point
   another name for focal point

Word History and Origins

Origin of principal focus¹

First recorded in 1825–35

Example Sentences

The distance from the principal focus to the center of the lens is the focal length or principal focal distance of the lens.

From Project Gutenberg

The point midway between the vertex V and center of curvature C is called the principal focus, F.

From Project Gutenberg

The principal focus of a concave mirror may be observed by holding the mirror in a beam of sunlight entering a darkened room.

From Project Gutenberg

The greater the refractive power of the lens, the greater the bending, and the nearer the principal focus to the lens.

From Project Gutenberg

If the light be placed beyond the principal focus of the lens, the rays will converge to a point some distance ahead of it.

From Project Gutenberg