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The refraction of glass

In Optics, Claudius Ptolemy (II century A.D.) presents several experiments to measure the effects of the refraction of a visual ray to the surface separating transparent materials, having different densities: air/water, air/glass and glass/water. The reconstruction proposes the second of these experiments, measuring the angles of refraction in the passage from air to glass, performed using a bronze plate with a graduated edge and a semi-cylinder of perfectly transparent glass (Optics, V, 14-18)

In the reconstruction, the visual ray which – in the Ptolemaic concept of the visual process – leaves the eye socket, is represented by a beam of light produced by a projector. A knob enables varying the projector's position and, consequently, the angle of incidence of the light beam on the flat semi-cylindrical glass surface. It is thus possible to verify the reliability of the experiment results reported by Ptolemy, concerning the perpendicular to the surface of air/glass separation (Optics, V, 18):


Angle of incidence                                Angle of refraction

10°                                                          7°

20°                                                          13° ½

30°                                                          19° ½

40°                                                          25°

50°                                                          30°

60°                                                          34° ½

70°                                                          38° ½

80°                                                          42°

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