“When light passes through different mediums speed changes but who provide acceleration to change velocity?”

This question was posted by Jishnu

Answer: Just imagine you are participating in a cross country race. When the road is good you can travel faster; but if the path is a marshy place, your speed will decrease. This is just an analogy. To understand it better, you should know that the way light travels in a medium is different from the way it travels in vacuum.