According to our current understanding of physics, light always travels at the same speed in a vacuum, which is approximately 299,792,458 meters per second (or about 186,282 miles per second), denoted as 'c.' This speed is a fundamental constant in nature.
However, when we consider light traveling through a medium other than a vacuum, such as air, water, or glass, its speed can be different. The speed of light in a medium is generally slower than its speed in a vacuum due to interactions between light and the atoms or molecules of the medium.
When light passes from one medium to another, such as from air to water or from air to glass, it undergoes a process called refraction. This change in medium causes the light to change direction, and its speed is altered as it interacts with the atoms or molecules in the new medium. The degree to which light is refracted depends on the properties of the medium, such as its refractive index.
It's important to note that the change in speed and direction of light due to refraction occurs within the medium itself and is not related to the speed of light in a vacuum. In a vacuum, light always travels at the same speed ('c') regardless of its direction.
So, the difference in light traveling at different speeds in different directions primarily arises when light encounters a medium with different optical properties, causing it to be refracted or slowed down. In a vacuum, however, the speed of light remains constant and independent of its direction of propagation.