In bifocal lenses, what does the term "back vertex" refer to?

In bifocal lenses, the term "back vertex" refers to the rear surface of the lens that significantly affects its optical power. The back vertex distance is the distance from the back vertex of the lens to the eye, and it is critical in determining how the lens will interact with light entering the eye. This part of the lens is crucial for ensuring that light is properly focused on the retina, which is especially important in bifocal designs where two different optical powers need to be effectively combined.

In bifocal lenses, the back vertex can have implications on how well the different portions of the lens work together and can influence the patient's visual experience. This makes understanding the concept essential for effectively fitting bifocals and ensuring optimal vision correction for the wearer.