抽象的

Ophthalmology is losing sight of optics

B Wieb Van Der Meer

An eye surgeon is ready to perform cataract surgery and replace a damaged crystalline eye lens by an implant. He is looking at the implant and has a suspicion that something is wrong. According to the specifications this lens should have a refractive index of 1.63 and a focal length of about 16 millimeters in an aqueous environment (with an index of about 1.33). This surgeon has a keen interest in optics, and knows that the ratio, focal-length-in-water-divided-by-that-in-air should be (1.63-1) divided by (1.63-1.33), so that the focal length in air should be about 8 millimeters, that is, about 1 centimeter. He holds the lens above the table and aims it at a ceiling lamp, moving the lens up and down until a sharp image of the lamp is formed on the surface of the table. The lens should be about a centimeter above the table in order to yield a clear image. However, he finds that this distance is much larger than that. He concludes that the company must have sent the wrong implant. The company is contacted, the surgery is postponed, and everything works out in the end. Knowledge of optics saved the day.