Light travels at an extraordinary speed in the emptiness of space. This speed, often considered a universal constant, prompts a natural question: does light maintain this same velocity when it passes through a medium like water?
The Core Question Answered
Light travels slower when it passes through water. While the speed of light in a vacuum is fixed, its speed changes in different materials. Individual photons do not inherently slow down; instead, the perceived reduction arises from their interactions within the medium. Light travels at approximately 300,000 kilometers per second in a vacuum, but slows to about 225,000 kilometers per second in water. Photons continue to move at their maximum speed between interactions, but the cumulative effect of these interactions creates a delay, making the overall wave propagate at a reduced speed.
Why Light’s Speed Changes
The apparent slowing of light in water stems from its interaction with water molecules. As an electromagnetic wave, light encounters electrons within these molecules. When a photon interacts with an electron, it is briefly absorbed, exciting the electron. The electron then re-emits a new photon, continuing the journey.
Each absorption and re-emission introduces a minuscule delay. This process does not dissipate energy in a transparent medium like water. Over countless such interactions, these tiny delays accumulate, resulting in an overall slower effective speed for the light wave.
Quantifying Light’s Speed in Water
To quantify how much a medium slows light, scientists use a measurement known as the refractive index (n). This index is defined as the ratio of the speed of light in a vacuum to its speed within the specific medium (n = c/v). For water, the refractive index is approximately 1.33. This indicates that light travels about 1.33 times slower in water than it does in a vacuum.
By contrast, air has a refractive index very close to 1.0003, meaning light travels almost as fast in air as in empty space. In a vacuum, the refractive index is exactly 1, representing light’s maximum possible speed.
Observable Effects of Light in Water
The change in light’s speed as it enters water produces several observable phenomena, most notably refraction. Refraction is the bending of light as it passes from one medium into another at an angle. This occurs because one side of the light wave enters the new medium and slows before the other, causing the wave to pivot.
For example, a straw in a glass of water appears bent or broken at the surface. Similarly, submerged objects can look distorted or shallower than their actual depth. This happens because our brains interpret light rays as traveling in straight lines, despite their bending upon entering or exiting the water.