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Light changes its speed in denser medium because refractive index of light is greater in the denser medium.
hope it helps u.....
•Light changes its speed in denser medium because in denser medium there are more number of particles as compared to rarer medium .

• So , in denser medium light starts colliding with the particles due to which its speed gets decreased as compared to less colliding in rarer medium like air

• Hope it helps ^_^