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On May 23, 1785, Benjamin Franklin (a name you have likely heard numerous times), wrote a letter to his friend George Whatley describing a new invention he had devised to solve a deeply personal problem. Interestingly, Franklin was nearsighted and also farsighted, which meant he needed two separate pairs of glasses: one for reading, and one for seeing things at a distance. Constantly switching between them, as one can imagine, had become exhausting. His solution turned out to be quite simple: cut both lenses in half and combine them into a single pair of glasses. This invention became known as the bifocal.
The problem it solved
Throughout human history, people have struggled with their vision. The ancient Romans, for instance, used glass spheres filled with water as magnifying tools. By the 13th century, rudimentary glasses had emerged in Italy. The challenge of simultaneously needing correction for both near and far vision — a condition called presbyopia — lacked an effective solution until Franklin's discovery.
The inconvenience Franklin described in his 1785 letter was this: he was attending dinners in France and struggling to see both the food on his plate and the faces of his dinner companions without constantly switching glasses. The fix he came to was practical: have two lenses fitted into a single frame, with the distance lens on top and the reading lens on the bottom.
How bifocals work
The physics behind bifocals is straightforward. Accommodation, the process by which the eye’s lens changes shape to focus on objects at different distances, weakens over time. With age, the lens loses flexibility and struggles to shift between near and far focus. Bifocal lenses compensate by doing the work the eye can no longer do on its own. The upper portion of the lens is ground to correct for distance vision, and the lower portion is ground to correct for near vision. Your eye naturally shifts downward when reading, moving into the near-correction zone without any conscious adjustment.
Modern bifocals have evolved considerably from Franklin's original design. Progressive lenses (sometimes called "no-line bifocals") are a seamless gradient between different focal strengths instead of a hard line between two distinct zones. Using anti-reflective coatings, photochromic lenses that darken in sunlight, and ultra-thin high-index materials, a greatly helpful optical device was produced.
Franklin's scientific legacy
The bifocal is just one of many scientific discoveries Benjamin Franklin has put his name to. He conducted systematic experiments on electricity, proved that lightning was electrical in nature, and invented the lightning rod, to name a few.
Today, more than a billion people worldwide wear corrective lenses of one kind or another. For millions who need correction at multiple distances (myself included), we can thank Franklin’s dinner-table frustration.
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