Light is a form of energy that moves through space in waves. When light travels from one transparent material, such as air, into another, like water, its path changes. This phenomenon, known as refraction, alters the direction of light as it crosses the boundary between these two different environments. This bending of light is responsible for many common visual effects we encounter in our daily lives.
How Water Slows Light Down
Light travels at varying speeds depending on the material it moves through. In a vacuum, light moves at its fastest, approximately 299,792 kilometers per second. However, when light enters a medium like water, it slows down, traveling at about 225,000 kilometers per second, which is roughly 75% of its speed in a vacuum. This reduction in speed occurs because water is optically denser than air. The molecules in water are more closely packed and interact with the light waves, causing a delay in their propagation.
Light travels as a series of wavefronts, which are like the crests of ocean waves. When these wavefronts encounter the surface of water at an angle, one part of the wavefront enters the denser medium and slows down before the rest of the wavefront. This asymmetrical slowing causes the entire wavefront to pivot, changing the direction of the light ray. The greater the difference in optical density between the two media, the more pronounced this change in speed and, consequently, the more the light bends.
The Angle of Light’s New Path
When light moves from a less optically dense medium, like air, into a more optically dense medium, such as water, it bends towards an imaginary line called the “normal”. This normal is always drawn perpendicular to the surface at the point where the light ray enters the water. The angle at which the light ray approaches the surface is known as the angle of incidence, while the angle at which it travels through the water after bending is the angle of refraction.
The relationship between these two angles is consistent for any given pair of materials, such as air and water. If light enters perpendicular to the surface (meaning its angle of incidence is zero), it will slow down but not change direction. However, at any other angle, the light ray will bend towards the normal, making the angle of refraction smaller than the angle of incidence. This predictable bending behavior is a direct consequence of the light waves adjusting their path due to the change in speed.
Everyday Examples of Light Bending
The bending of light at the surface of water creates several common visual illusions. One familiar example is a straw or spoon appearing bent or broken when placed in a glass of water. The light reflecting off the part of the straw underwater bends as it exits the water and enters the air, causing our eyes to perceive it in a different position than its actual location. Since our brains are accustomed to interpreting light rays as traveling in straight lines, this bending creates the illusion of a distorted object.
Similarly, objects submerged in water, such as a fish in a pond or the bottom of a swimming pool, often appear shallower or closer than they truly are. Light rays reflecting off these underwater objects travel from the water into the air, bending away from the normal as they speed up. When our eyes receive these refracted rays, our brains project them backward in a straight line, leading us to believe the object is at a shallower depth. This effect explains why judging the actual depth of water can be challenging.