Wi-Fi radio waves spread out as they travel, so signal strength generally falls with distance from the router. Floors and walls add further obstacles. Building materials can absorb or reflect some of the wave energy. Concrete, metal, foil-backed insulation and even water-containing materials can weaken signals significantly. The route upstairs may also involve several boundaries and awkward angles, creating regions where reflected waves interfere or the direct signal is weak. Moving a router into a more central, open position can improve coverage because it reduces distance and the amount of material the waves must cross.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Waves transfer energy and information without transferring matter from source to receiver overall. Frequency tells us how many oscillations pass each second, wavelength is the spacing between matching points, and wave speed links them through v = fλ. Different frequencies interact with materials differently, which is why engineers can choose particular parts of the electromagnetic spectrum—or particular sound frequencies—for sensing, communication, heating or imaging.