Some facial-recognition systems can work even when the room looks completely dark to you. They do not have to rely only on visible light. A device can project infrared radiation onto a face and detect the radiation reflected back. Infrared is part of the electromagnetic spectrum but has wavelengths longer than visible red light, so human eyes cannot see it. By analysing patterns or measuring depth, the phone can build information about the shape of the face and compare it with stored data. The physics begins with emission, reflection and detection of electromagnetic waves; software then performs the pattern matching.

This is also a design problem. Cameras, glasses, screens, road signs and medical scanners all depend on controlling how electromagnetic waves are emitted, absorbed, reflected or refracted. A familiar object can therefore be a practical example of the same wave rules studied in class.

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.