Light travels fastest through a vacuum. In a vacuum, such as empty space, light can travel at its maximum speed, which is approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. This speed is commonly denoted as the "speed of light in a vacuum" and is denoted by the symbol "c" in physics equations.
When light travels through other mediums, such as air, water, glass, or any other transparent material, its speed is slower than the speed of light in a vacuum. The speed of light in a given medium is determined by its refractive index, which is a measure of how much the medium slows down the speed of light compared to a vacuum. The refractive index of a medium is typically greater than 1, meaning light travels slower in that medium compared to a vacuum.
It's worth noting that the speed of light in different materials can vary, and it depends on various factors such as the composition, density, and temperature of the medium. However, in all cases, the speed of light in a medium is always less than or equal to the speed of light in a vacuum.