Corpuscles in a vacuum moved unimpeded, and they slowed only slightly in air. Ordinary glass cut their speed approximately in half. Newton's theory, for example, predicts light will bend at a glass-air interface in the direction opposite to that actually observed. Still, Newton's corpuscular theory made it easy to visualize light-matter interactions, and it provided some answers to basic optics questions. The speed of light is a constant.
Light slows down when it encounters matter.
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