An oscillation does not need to remain localized—it can propagate through space. This occurs when multiple oscillators are coupled together. Examples of waves include sound and water waves, where the coupling is caused by forces between atoms and molecules. Both are examples of mechanical waves, which require a medium (like air or water) for propagation.
In contrast, there are also waves that can travel through a vacuum. A well-known example is electromagnetic waves, which maintain themselves by continuously generating alternating electric and magnetic fields. These will be discussed in a later chapter.
Waves can be made visible using a wave machine—a device consisting of many interconnected oscillators. When one oscillator is displaced up and down, the resulting motion spreads along the chain, forming a wave.
A mechanical wave can also be visualized using a rope attached to a wall. If you momentarily move the free end of the rope, a disturbance is created that travels along the rope until it reaches the wall. The wall acts as a fixed end, and the incoming wave exerts a force on it. According to Newton's third law, the wall exerts an equal and opposite force back on the rope. As a result, the direction of displacement is inverted, and the reflected wave experiences a phase shift of $180^\circ$ or $\pi$.
We will later encounter other situations where such a phase shift occurs.
At an open end, however, no such phase shift occurs. This can be demonstrated with a rope hanging vertically from an elevated point. When a wave is introduced, it travels downward and reflects back with the same phase to the origin.