<P> The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy . They viewed light as being an atomic entity equivalent to energy . </P> <P> René Descartes (1596--1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al - Haytham and Witelo as well as the "species" of Bacon, Grosseteste, and Kepler . In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium . Descartes arrived at this conclusion by analogy with the behaviour of sound waves . Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media . </P> <P> Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as the start of modern physical optics . </P> <P> Pierre Gassendi (1592--1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s . Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum . He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source . One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines . He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether . </P>

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