Final answer:
In optics, the notation of the axis when wearing a lens refers to the optical axis, which is the path along which light rays travel before and after passing through the lens, and is critical for locating images using ray diagrams.
Step-by-step explanation:
When wearing a lens and looking out through it, the notation of the axis runs from the lens itself along the direction of the outgoing light rays, generally described as the optical axis. For a converging lens, a ray of light entering parallel to the optical axis will converge at the focal point on the other side of the lens. Conversely, a ray passing through the focal point before it enters the lens will exit parallel to the optical axis. This behavior of light is critical when locating the image of an object, such as an arrow, viewed through the lens. To completely locate the extended image, two principal rays from the top of the object are often used: one parallel to the optical axis and another either through the lens's center or focal point.
Sign conventions for lenses, such as focal length and image orientation, must be followed when applying the thin-lens equation to ensure the accurate positioning of images. The image is oriented along the optical axis, which includes a vertical dimension referenced to the lens's principal axis. By following the symmetrical properties of lenses and understanding how light interacts with them, one can predict and draw the path of light rays and the resulting image's position and orientation.