The speed of light is a constant, regardless of the medium it travels through. However, the speed at which light appears to travel can change depending on the medium's refractive index.

Here's the breakdown:

* The speed of light in a vacuum is the fastest possible speed, denoted by 'c'. This is approximately 299,792,458 meters per second.

* When light enters a medium other than a vacuum, its speed slows down. This is because the light interacts with the atoms of the medium, which causes it to change direction and travel a longer path.

* The refractive index of a medium is a measure of how much light slows down when it passes through it. A higher refractive index means a slower apparent speed of light.

Therefore, the minimum speed of light is achieved when it travels through a medium with the highest possible refractive index. This is achieved in certain materials like diamonds, where the refractive index is around 2.42. This means that light travels approximately 1.22 times slower in diamond compared to a vacuum.

It's important to remember that this is the apparent speed of light. The fundamental speed of light (c) remains constant.