Your phone can work out its position by receiving precisely timed radio signals from navigation satellites. Each signal tells the receiver when it was sent and which satellite sent it. Radio waves travel at the speed of light. By measuring how long a signal took to arrive, the receiver can estimate its distance from that satellite. Distances from several satellites allow the receiver to calculate a three-dimensional position and correct its clock. This process is closely related to trilateration. Tiny timing errors would create large position errors because light travels about 300,000 kilometres every second, so GPS depends on extremely accurate clocks.

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.