The distance of 16.67 mm in the Reduced Eye model is associated with which optical law?

• A. Snell's law
• B. Knapp's law
• C. Fermat's principle
• D. Descartes' law

Answer: (B)

In the reduced-eye model, the retina sits at a nearly fixed distance behind the eye’s posterior focal point—about 16.67 mm. This comes from simplifying the eye to a single equivalent refracting element, so the image of distant objects lands on the retina at a predictable plane. Knapp’s law captures this idea: the distance from the posterior focal point to the retina remains essentially constant in this model, which is why that specific 16.67 mm value is used.

This isn’t about how light bends at interfaces (that’s Snell’s or Descartes’ refractive law) or about the general principle of light following the path of least time (Fermat’s principle). It specifically describes the fixed retinal distance in the simplified, paraxial eye model.