Walls can stop or attenuate electromagnetic waves (including light) from passing through them through a process called absorption, reflection, and scattering.
Absorption: When electromagnetic waves encounter a wall, the wall's material absorbs some of the energy carried by the waves. The absorbed energy is converted into other forms, such as heat, within the wall. The extent of absorption depends on the properties of the wall material and the frequency of the electromagnetic waves. Some materials, like metals, can effectively absorb and dissipate electromagnetic energy, while others, like concrete or wood, may have varying degrees of absorption depending on their composition.
Reflection: When electromagnetic waves encounter a wall, a portion of the energy can be reflected back. The reflective properties of a wall depend on its surface characteristics and the angle of incidence of the waves. Smooth and polished surfaces, such as those found in mirrors, are particularly effective at reflecting light waves. The reflection of electromagnetic waves allows them to bounce off the surface of the wall and change direction.
Scattering: Scattering occurs when electromagnetic waves encounter irregularities or rough surfaces on the wall. These irregularities cause the waves to scatter in different directions. Scattering can disperse the energy of the waves and prevent them from maintaining their original direction or coherence. The scattering of light waves, for example, can result in diffused or scattered light instead of a direct beam.
The combination of absorption, reflection, and scattering leads to the attenuation of electromagnetic waves as they interact with walls. The specific behavior of waves depends on factors such as the material properties of the wall, the frequency of the waves, and the angle of incidence. In many cases, walls are designed to minimize the transmission of electromagnetic waves by maximizing absorption, reflection, or scattering, depending on the desired outcome. However, it's important to note that no wall is completely impenetrable, and some amount of electromagnetic energy can still pass through or around them, depending on the circumstances.