When light traveling through air encounters the surface of water, it undergoes refraction. This phenomenon describes the change in the light wave’s direction as it moves from one transparent medium, like air, into another, such as water. The boundary between the two substances acts as a transition point where the light ray’s properties are altered. This change in path is a predictable physical response to the new material the light is entering.
Defining Refraction and the Change in Speed
The physical mechanism causing light to bend is a measurable change in its speed as it enters a new medium. Light travels fastest in a vacuum, but it slows down when passing through any material, including air and water. Water is considered an optically denser medium than air, causing light to slow significantly.
This reduction in speed creates the bend, particularly when the light ray strikes the surface at an angle. Imagine the light ray as a wavefront, like a line of marching soldiers transitioning from a paved road to mud. The first part of the wavefront to hit the water slows down while the rest of the wave is still moving faster in the air. This differential slowing causes the wavefront to pivot, which is observed as the light ray changing direction.
The light ray bends toward the “normal,” an imaginary line drawn perpendicular to the water’s surface at the point of incidence. If a light ray enters the water directly along the normal, all parts of the wavefront slow down simultaneously, and the light does not change direction, only speed. The greater the angle at which the light hits the surface, the more pronounced the bending becomes.
Quantifying the Degree of Bending
The extent to which light slows down and bends in a medium is quantified by the Index of Refraction (IOR). The IOR is a ratio comparing the speed of light in a vacuum to its speed within that material. For example, the IOR for water is approximately $1.33$, meaning light travels $1.33$ times slower in water than it does in a vacuum.
The IOR of the two materials, air and water, determines the maximum potential for bending. The precise angle of the bend is governed by the angle at which the light strikes the surface, known as the angle of incidence.
The relationship between the indices of refraction and the angles is described by Snell’s Law. This principle establishes that a steeper angle of incidence results in a greater observed change in the light’s path. Therefore, light hitting the water’s surface at a shallow angle will show more dramatic bending than light hitting the surface close to the perpendicular normal line.
How Refraction Distorts Vision
Refraction at the water’s surface creates visual illusions because the human brain assumes light always travels in a straight line. When we look at an object partially submerged, the light rays reflecting off the submerged portion bend as they exit the water and enter the air before reaching the eye. The brain then automatically traces these refracted rays straight back to a false location.
This effect causes objects like a stick or a straw to appear bent or “broken” at the water line, as the submerged part seems shifted from its true position. Another common visual distortion is apparent depth. Objects underwater, such as a fish or the bottom of a pool, appear closer to the surface than they are in reality.
The light rays originating from the object bend away from the normal as they move from the denser water to the less dense air. The brain interprets these rays as coming from a shallower point, making the object’s apparent depth less than its actual depth. This distortion requires spear fishermen to aim below the apparent location of a fish.