when a light ray travel from a rarer to denser medium ,the light bend toward the normal . How corpuscular theory reveals that speed of light in denser medium is greater than speed of light in rarer medium.
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Sir Isaac Newton came up with the corpuscular theory of light in order to explain the phenomena of reflection and refraction.
He postulated that as a ray of light (which is composed of tiny particles called corpuscles) falls upon a comparatively denser medium, they experience attraction. This attraction encountered by light particles is more than what is experienced in a rarer, due to the availability of more medium particles per volume (denser - tightly packed). So, he imagined that the corpuscles would accelerate in what can be called a gravitational forces exerted by the medium particles. Thus, the speed of light would become more in a denser medium according to Newton's Corpuscular theory.