The Phenomenon of Light Refraction
Objects appear bent or broken in water due to light refraction. Refraction is the bending of light as it passes from one transparent medium to another, such as from air into water or vice versa. This bending occurs because light changes its speed when moving between materials with different optical densities.
How Light Bends at the Water's Surface
When light rays travel from a submerged object (e.g., a straw) through water and then into the air to reach our eyes, they change direction at the water's surface. Light slows down in water and speeds up in air, causing it to bend away from the 'normal' (an imaginary line perpendicular to the surface) as it exits the water.
The Brain's Interpretation and Apparent Position
Our brains are wired to assume that light travels in straight lines. When the bent light rays from the submerged object reach our eyes, our brain traces these rays back in a straight path. This re-traced path creates an optical illusion, making the object appear to be in a different position than its actual location, thus giving it a 'bent' or 'broken' appearance.
Practical Example: The Bent Straw Illusion
A classic example is observing a straw placed in a glass of water. The part of the straw above the water appears normal, while the submerged part seems displaced or angled differently, creating the illusion that the straw itself is bent. This visual distortion is entirely a result of how light behaves and how our brain interprets it.