procedure:
1. measure the height of bucket and evenly spaced it into 5 different heights
2. fill the bucket with water
3. put a coin into the bucket
4. move a coin up and down beside the bucket ( in air) until the coins are about the same size
5. measure the height between the coin (in air) and the surface of water
my question is
as i plot the graph as the height of water against the height of coin, why the slope is equal to the refractive index of water?