Abstract
In Chap. 2 of his Treatise on Light, Huygens demonstrated geometrically how the law of reflection follows naturally from the wave theory of light. This he did by first considering a wave-front incident at an oblique angle on a reflecting surface. As this wave-front encountered successive points on the surface, they emitted spherical partial waves which propagated away from the surface of the mirror. Then, by drawing a tangent line to these spherical wave-fronts, he was able to reconstruct the outgoing wave, whose direction of propagation was found to obey the law of reflection. With this success, Huygens now turns to the law of refraction. In this case, the incoming wave-front proceeds into a transparent material. This raises the question: why are some some materials transparent, while others reflect light? The discussion turns inevitably to the microscopic structure and composition of matter. Do you find Huygens’ ideas plausible? If not, then what do you think might account for the diverse optical properties of materials?
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