Question

when you immerse an empty plastic bottle in bucket of water it comes above the water surface why does this happen​

Answer 1

only an one word answer and that is BENDING OF LIGHT DUE TO REFRACTIvE INDEX

Answer 2

Objects having density less than that of the liquid in which they are immersed, float on the surface of the liquid. If the density of the object is more than the density of the liquid in which it is immersed then it sinks in the liquid. The air ,inside the bottol , makes the density of the total substance (i.e. the bottle) lighter then water . So as a result it floats on water .