When we say that the speed of light is constant, we are specifically referring to the speed of light in a vacuum. In a vacuum, which is a space devoid of matter, light travels at a constant speed. This speed is approximately 299,792,458 meters per second (or about 186,282 miles per second) in the commonly used units.
It is important to note that the speed of light can vary when it propagates through different mediums, such as air, water, or glass. When light travels through a medium other than a vacuum, it interacts with the atoms or molecules of that medium, which can cause the light to slow down. This phenomenon is described by the concept of the refractive index, which measures how much a medium can slow down light compared to its speed in a vacuum.
So, while the speed of light in a vacuum is constant, it can be different in different mediums due to interactions with matter. However, the constancy of the speed of light in a vacuum is a fundamental principle of physics and has significant implications in our understanding of the universe, particularly in the theory of relativity.