Lenses

• What Is the Difference Between Paraxial and Marginal Rays?
  Paraxial rays lie near an optical axis at small angles; marginal rays pass through the aperture edge and may reveal lens aberrations.
• Can a Concave Lens Form a Real Image, and What Is Its Magnification?
  A concave lens can form a real image only from converging incident rays; use signed lens distances to find its image and magnification.
• Are Real Images Always Inverted and Virtual Images Always Erect?
  Real images are inverted and virtual images erect for a real object imaged by one standard lens or mirror, but the rule has limits.
• 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.