為何會有resonance?

Any object will have a "natural frequency of vibration" under which the object vibrates with maximum amplitudes. This "natural frequency" depends on the mass and geometry of the object.
When the object is subject to an external vibration, if the frequency of external vibration equals to the natural frequency of the object, energy transfer between the external vibration and the vibrating object reaches a maximum. This is how "resonance" occurs.

In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonance frequencies (or resonant frequencies). At a resonant frequency the frequency of oscillation does not change with changing amplitude. Therefore, at these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy. When damping is small, the resonance frequency is approximately equal to the natural frequency of the system, which is the frequency of free vibrations. Resonant phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance, Nuclear Magnetic Resonance (NMR), Electron Spin Resonance (ESR) and resonance of quantum wave functions. Resonant systems can be used to generate vibrations of a specific frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies.

Resonance was discovered by Galileo Galilei with his investigations of pendulums beginning in 1602