An Optical Method for Demonstrating the Point of Inflecton of Curves
A. Adler Hirsch · Mathematics Teacher Learning and Teaching PK-12 · 1966
An Inflection point, p, of a plane curve, AB, in the diagram, may be demonstrated optically by noting the disappearance of the faint secondary reflection (ghost image) from the lower front surface of an ordinary glass mirror, M, which straddles the curve perpendicularly. Perpendicularity is attained when the main reflected image, i, appears to blend smoothly at the bottom of the minor with the preceding arc. At all other points, such as r1 and r2, secondary images, s1 and s2, form in the mirror. These are always displaced from the primary images, i1 and i2, in a direction toward the concavity of the arc.