The statement that an atom is 99% empty space refers to the fact that atoms are primarily composed of subatomic particles, such as protons, neutrons, and electrons, which are surrounded by vast regions of empty space. However, it is important to note that the empty space within an atom does not imply that atoms themselves are transparent or allow us to see through solid objects like walls.
The reason you cannot see the other side of a wall is due to the way light interacts with matter. When light encounters an object, such as a wall, it interacts with the atoms or molecules that make up the material. The interaction between light and matter depends on several factors, including the properties of the material and the wavelength of the light.
In the case of a wall, the atoms or molecules in the material absorb and scatter the incoming light. The absorbed light energy can be converted into other forms, such as heat, or it can be re-emitted in various directions. However, the light is generally scattered in all directions rather than passing straight through the wall. This scattering of light prevents us from seeing objects on the other side of the wall.
Furthermore, most common materials, such as concrete or brick, are opaque, meaning they do not allow light to pass through them easily. The arrangement of atoms and molecules in these materials is such that they absorb or reflect a significant portion of the incident light, making it difficult for light to propagate through the wall.
In some cases, transparent or translucent materials, like glass or certain plastics, allow light to pass through more easily due to their molecular structure. However, even in these cases, the interaction between light and matter still occurs, albeit to a lesser extent, and the transmitted light may be distorted or weakened as it passes through the material.
So, while atoms may be predominantly empty space, the interaction of light with matter at the atomic level and the properties of the materials involved determine whether or not we can see through a wall or any other object.