Place a coin on the bottom of the teacup and position your eye so that the edge of the cup covers it.

Place a coin on the bottom of the teacup and position your eye so that the edge of the cup covers it. if you pour water into a cup, the coin will be visible. why?

Water has a different refractive index than air. In relation to these two media, water is optically denser, therefore, after “emerging” from the water, the angle in air will be greater than in water (and in water the angle is the same when we looked at a coin without this liquid), because the angle has increased, then you can see this coin.