Here's why:

* The Speed of Light: The speed of light in a vacuum is the fastest speed possible in the universe, approximately 299,792,458 meters per second. We often call this value "c".

* Refractive Index: The speed of light slows down when it passes through a medium like a solid, liquid, or gas. This is because the light interacts with the atoms in the medium, causing it to change direction and slow down. The amount of slowing down is determined by the refractive index of the material.

* Higher Density = Slower Light: Solids are generally denser than liquids and gases. This means there are more atoms packed together in a smaller space, leading to more interactions with light and a slower speed.

Exceptions:

While light generally travels slower in solids, there are some special cases where light can travel faster in certain solids than in a vacuum. This occurs in some materials with a very specific structure, where light can interact with the electrons in a way that allows it to "tunnel" through the material faster than it would in a vacuum. This phenomenon is called "superluminal propagation."

In summary:

* Light travels slowest in a vacuum.

* Light travels slower in solids than in liquids or gases, but faster than in a vacuum in specific cases.