I am doing independent research to compute eye strain and I hit a problem. Hope someone can help. This is the situation. Work done to pull a spring is ½kx2. The human lens is like a spring. Linear, and obeys Hooke’s law. Work done to stretch the lens is also ½ kx2 (k=spring constant of lens, x = lens stretch) Since stretching of the lens causes eye strain, I created an equation: Eye strain = work done = ½ k x2 So far so good. What follow is tricky. The minute stretching of the lens correlates to change in viewing distance in real life but in reverse order, i.e. lens stretch increases, viewing distance decreases. Instead of measuring lens stretch to compute eye strain, which is almost impossible, I want to measure viewing distance to compute eye strain. If the change in viewing distance in real life is (d) Eye strain = work done = ½ k (1/d2) Question 1: Can I legally inverse (d) to reflect viewing distance is the inverse of lens stretch (x)? (Looking at the equation, it looks correct. Eye strain is the inverse of distance focus.) Question 2: The SI unit for work done is N/m. This is not the case after I inverse (d). It becomes N/m3 . How do I resolve the SI units or do I need to? Thanks for helping.