Bifocal glasses have two parts in each lens: the upper part is made for seeing things far away, and the lower part is made for reading. You can usually see a line across the lens where the two strengths meet. People wear them when one pair of glasses is no longer enough for both distances — often later in life. You can say bifocal as a describing word ('bifocal lenses') or talk about a pair of bifocals. Progressive lenses are a different, later design: many strengths blended together, with no sharp line. People mix the two names up, but opticians keep them apart. The American Benjamin Franklin is usually given the credit for inventing bifocals, in the 1700s.

雙光眼鏡、雙焦鏡片（上遠下近，中間有分界）。漸進多焦是另一種，沒有那條線。