Speed of Light. Anybody with good knowledge of Physics. Pls help?

As we all know, speed of light is the universal physical constant. It is always measured to be same with reference to anybody, anything...(theory of general relativity). But how can we say that, or how does it happens that speed of light decreases within objects of higher refractive index, like it is 75% only in... show more As we all know, speed of light is the universal physical constant. It is always measured to be same with reference to anybody, anything...(theory of general relativity). But how can we say that, or how does it happens that speed of light decreases within objects of higher refractive index, like it is 75% only in water. More dense the object, lesser the speed. Does time dilates in water (what the hell). What am I missing???
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