Refraction

• Earth Curvature and Refraction, Explained Simply
  Estimate Earth’s curvature drop with distance, see how atmospheric refraction changes it, and use a simple horizon formula with worked examples.
• Can the Focal Length of a Convex Lens Be Negative?
  A convex lens usually has positive focal length in air, but it can have negative focal length if its surroundings make it diverge light.