focal length
phys. Distance between the center of a lens and either focal point; the distance from a mirror to its focal point. དྭངས་ཤེལ་གྱི་ལྟེ་བ་ནས་འདུ་གནས་གང་རུང་དབར་གྱི་བར་ཐག མེ་ལོང་ནས་དེའི་འདུ་གནས་དབར་གྱི་བར་ཐག
focal plane
phys. Plane, perpendicular to the principle axis, that passes through a focal point of a lens or mirror. For a converging lens or a concave mirror, any incident parallel rays […]
focal point
phys. For a converging lens or a concave mirror, the point at which rays of light parallel to the principal axis converge. For a diverging lens or a convex mirror, […]
focus (foci)
phys. (a) For an ellipse, one of the two points for which the sum of the distances to any point on the ellipse is a constant. A satellite orbiting Earth […]