Convex vs Concave Lenses

العدسات المحدبة والمقعرة

A convex (converging) lens is thicker at its center than at its edges and converges parallel light rays toward a real focal point. A concave (diverging) lens is thinner at its center than at its edges and diverges parallel light rays as if from a virtual focal point.

Quick Decode

A convex lens is thicker in the middle and brings parallel light rays together at a real focal point. A concave lens is thinner in the middle and spreads parallel rays apart, so they appear to come from a virtual focal point.

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References (1)
  1. Keating, M. P. Geometric, Physical, and Visual Optics (2nd ed., Chapters 4 & 5) & AAO BCSC Section 3 (Clinical Optics).