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Bow-shaped caustics from conical prisms : a 13th Century account of rainbow formation from Robert Grosseteste’s De iride.

Harvey, Joshua and Smithson, Hannah and Siviour, Clive and Gasper, Giles E. M. and Sonnesyn, Sigbjorn O. and Tanner, Brian K. and McLeish, Tom (2017) 'Bow-shaped caustics from conical prisms : a 13th Century account of rainbow formation from Robert Grosseteste’s De iride.', Applied optics., 56 (19). G197-G204.


The rainbow has been the subject of discussion across a variety of historical periods and cultures, and numerous optical explanations have been suggested. Here, we further explore the scientific treatise De iride [On the Rainbow] written by Robert Grosseteste in the 13th century. Attempting to account for the shape of the rainbow, Grosseteste bases his explanation on the optical properties of transparent cones, which he claims can give rise to arc-shaped projections through refraction. By stating that atmospheric phenomena are reducible to the geometric optics of a conical prism, the De iride lays out a coherent and testable hypothesis. Through both physical experiment and physics-based simulation, we present a novel characterization of cone–light interactions, demonstrating that transparent cones do indeed give rise to bow-shaped caustics—a nonintuitive phenomenon that suggests Grosseteste’s theory of the rainbow is likely to have been grounded in observation.

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Publisher statement:Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Date accepted:19 April 2017
Date deposited:08 August 2017
Date of first online publication:30 June 2017
Date first made open access:No date available

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