Final answer:
A standing wave occurs when an object vibrates at its natural frequency, resulting in a wave that appears motionless. This is observed in systems driven at resonance, such as the oscillation of a ball on an elastic band or strings of musical instruments.
Step-by-step explanation:
A standing wave can occur when an object is made to vibrate at its natural frequency. When this happens, the wave seems to be motionless. An example is the paddle ball suspended from a finger; as you move your finger up and down at different frequencies, the ball will oscillate with varying amplitudes. When you vibrate the ball at its natural frequency, it resonates, and the ball's oscillations increase dramatically. However, if the driving frequency is higher than the natural frequency, the ball's oscillations become smaller until they nearly disappear.
The same concept applies to musical instruments like a piano or guitar, where strings vibrate and produce sound at their natural frequencies. Similarly, standing waves can be observed in a glass of milk in a refrigerator where the surface waves created by vibrations from the refrigerator motor oscillate but do not seem to move across the surface, demonstrating the standing wave phenomenon through superposition and interference of waves.