In a vacuum, light always travels at the same speed, which is approximately 299,792,458 meters per second. However, light can travel at slower speeds when passing through a medium such as air, water, or glass. This phenomenon is known as the speed of light's reduction due to the refractive index of the medium.
When light slows down while passing through a medium, it typically experiences a change in direction as well. This change in direction is known as refraction. The amount of refraction depends on the refractive index of the medium and the angle at which the light enters the medium.
Regarding changes in brightness, when light travels through a medium with a lower refractive index, it generally remains just as bright as when it entered the medium. However, if the medium has a higher refractive index, the light may scatter or be absorbed, leading to a decrease in its intensity or brightness.
The specific amount by which the speed of light slows down in a medium depends on the properties of the medium itself. For example, in air, light slows down by a very small amount compared to its speed in a vacuum. In other substances like water or glass, the slowdown is more noticeable.
It's important to note that while light can be slowed down in a medium, it still travels at extremely high speeds compared to everyday objects. The differences in speed and brightness are typically subtle unless you're dealing with highly specialized experiments or situations involving exotic materials.