The Swimming Pool Coin Illusion: Why Objects Appear Deeper Than They Are

The answer lies in the way light travels from the coin to your eyes. As light passes from one medium to another (in this case, from air to water), it changes speed and direction. This phenomenon is known as refraction.

When you look at a coin at the bottom of a pool, the light rays that come from the coin are refracted as they pass from the water to the air. This refraction causes the coin to appear closer to the surface of the water than it actually is. This is the same reason that a straw in a glass of water appears to be bent at the water line.

When you dive for the coin, you're actually aiming for a spot that is slightly above the coin's actual location. This is because you're perceiving the coin's location based on the refracted light rays. By aiming slightly higher, you're able to compensate for the refraction and reach the coin.