You can watch a video on your phone even when the Wi-Fi router is in another room. The information reaches your device using electromagnetic waves, usually at microwave frequencies. These waves are part of the same electromagnetic spectrum as visible light, although their wavelengths and frequencies are very different. When a Wi-Fi signal reaches a wall, several things can happen. Some of the wave can pass through the material, some can be reflected and some of its energy can be absorbed. The amount transmitted depends on the frequency of the signal and the thickness and composition of the wall. This is why Wi-Fi can behave differently in different buildings. Materials such as plasterboard may allow a useful signal through, while thick concrete or metal can weaken it considerably. Water-containing materials can also absorb some microwave energy. Wi-Fi waves can reflect from floors, ceilings, furniture and other surfaces. A signal may therefore reach your phone along several different paths. When these waves overlap, they can interfere with one another. Modern Wi-Fi systems use sophisticated electronics and signal processing to operate in this complicated environment. Wi-Fi is only one use of electromagnetic radiation. Radio broadcasts, mobile phones, infrared remote controls, visible light and X-rays all involve electromagnetic waves with different frequencies and wavelengths. So when a message passes through a wall to your phone, the wall has not become invisible to the signal. Some electromagnetic energy has simply been transmitted through the material strongly enough for the receiver to detect and decode it.