What do you call the points where the principal planes of a lens intersect the axis?

The points where the principal planes of a lens intersect the optical axis are known as principal points. These points are essential in understanding the lens's optical properties, as they relate to how light is focused and the path it takes through the lens system.

In terms of optical design, principal points are crucial for determining the effective focal length and for analyzing the overall geometry of the lens. When light rays pass through a lens, they bend according to the curvature and orientation of the lens, and the principal points indicate the locations where these rays effectively converge to create an image.

Understanding the principal points helps in the calculation and design of various optical instruments, including eyeglasses and cameras, as it allows for accurate prediction of focus and image distortion. This foundational concept is key when evaluating lens performance in practical applications.