Does light travel more quickly through water or through air?

Light, a form of electromagnetic radiation, travels at its fastest possible speed in a vacuum, a universal constant known as 'c', which is approximately 299,792,458 meters per second. However, when light passes through any medium, such as air or water, its speed decreases. This phenomenon is due to the interaction of light with the particles in the medium.

The extent to which light slows down in a medium is quantified by its refractive index. The refractive index is a ratio of the speed of light in a vacuum to the speed of light in that particular medium. A higher refractive index indicates a slower speed of light in the medium. Air has a refractive index very close to that of a vacuum, around 1.0003, meaning light slows down only slightly in air. In contrast, water is a denser medium with a refractive index of about 1.333.

Because water is significantly denser than air, light encounters more particles and interacts with them more frequently, causing it to be absorbed and re-emitted by the atoms within the medium. This continuous process effectively slows down the apparent speed of light. Therefore, light travels faster through air than it does through water. While light travels at almost the same speed in air as it does in a vacuum, its speed in water is reduced to approximately 225,000,000 meters per second. This difference in speed is what causes fascinating optical effects like refraction, where light bends as it passes from one medium to another.