Yes, the velocity of light decreases as it travels through a given medium, such as water, glass, or air. This phenomenon is known as "refraction." The reduction in the speed of light in a medium is due to the interaction between light and the atoms or molecules of that medium.
When light travels through a vacuum, it propagates at the speed of light in a vacuum, which is approximately 299,792 kilometers per second (about 186,282 miles per second). However, when light enters a material medium, it interacts with the atoms or molecules of that medium. The electromagnetic waves of light cause the charged particles in the atoms or molecules to oscillate, and these oscillations affect the propagation of light through the material.
In a transparent medium, such as glass or water, the interaction between light and the atoms/molecules results in a delay as the electromagnetic waves are absorbed and re-emitted by the charged particles. This delay reduces the overall speed at which light can travel through the medium.
The reduction in speed can be quantified using the refractive index of the medium. The refractive index (n) is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v):
n = c / v.
Since the speed of light in the medium (v) is less than the speed of light in a vacuum (c), the refractive index is always greater than 1 for any transparent medium.
Different materials have different refractive indices, and this property is why light changes direction (bends) when it travels from one medium to another at an angle. This bending of light is what causes various optical phenomena, such as the refraction of light in a prism, the formation of rainbows, and the magnification of objects by lenses.
In summary, the velocity of light decreases when it passes through a medium due to interactions between light and the atoms or molecules in that medium, resulting in the phenomenon of refraction.