Light, in a vacuum, always travels at the same speed, which is approximately 299,792,458 meters per second (or about 186,282 miles per second). This value is commonly denoted as the speed of light, symbolized by the letter "c." According to our current understanding of physics, this speed is a fundamental constant and is considered to be the maximum attainable speed in the universe.
However, when light travels through a medium other than a vacuum, such as air, water, or glass, its speed can be significantly different from the speed of light in a vacuum. This discrepancy arises due to the interaction between light and the atoms or molecules of the medium.
In a medium, light interacts with charged particles, such as electrons, within the atoms or molecules. These interactions cause the light to be absorbed and re-emitted by the particles, effectively slowing down its overall speed. This process is known as "absorption and re-emission" or "scattering."
The speed of light in a medium is typically slower than its speed in a vacuum and is characterized by the medium's refractive index. The refractive index of a medium indicates how much the speed of light is reduced compared to its speed in a vacuum. It is a dimensionless quantity and is specific to each material.
The relationship between the speed of light in a vacuum (c), the speed of light in a medium (v), and the refractive index (n) of the medium is given by the equation:
v = c/n
This equation shows that the speed of light in a medium is inversely proportional to the refractive index of the medium. Hence, different materials with different refractive indices will cause light to travel at different speeds.
It's important to note that the speed of light in a medium is still very high compared to everyday objects and is generally considered "fast" from our human perspective. However, the difference in speed can be significant enough to have practical implications, such as the bending of light as it passes from one medium to another (refraction) or the creation of optical phenomena like rainbows.