Definition
Resonance is a physical effect where an object's vibration grows dramatically when an outside force is applied at the same rhythm as the object's own natural vibration. With perfect timing, the same amount of force can create a much bigger swing. But friction slowly drains energy, so the vibration doesn't grow forever.
With the right timing, a small push becomes huge
Picture a kid on a playground swing. If you give their back a light push at the exact moment they reach the highest point and pause, the swing climbs higher and higher with very little effort. Push at random times, though, and you actually get in the swing's way.
Every object has a special rhythm it likes to move at. In physics, this is called its natural frequency (the speed at which an object vibrates on its own). Wine glasses, bridges, even the organs inside your body each carry a rhythm of their own.
When the rhythm of an outside force matches an object's natural frequency exactly, energy builds up bit by bit. This sudden amplification of vibration is called resonance.
From musical instruments to hospital scanners
We can enjoy beautiful music thanks to resonance. When you pluck a guitar string, the string alone makes only a tiny sound. But the air inside the guitar body vibrates along with it and creates a rich, full sound. The resonator tubes of a xylophone and wind instruments all boost their sound the same way.
Resonance also works hard in the world of radio waves and magnets. Turning a radio dial matches the radio circuit's frequency to the station you want, so it picks out just that signal.
The magnetic resonance imaging (MRI) used for detailed hospital exams relies on resonance too. In a strong magnetic field, hydrogen nuclei take on a certain rhythm. The scanner sends radio waves tuned to that rhythm to shake the nuclei, then reads the signals they give off as they settle back, and uses them to image a slice of your body.
To be more precise: energy gets trapped
To be more precise, resonance is a state where energy from outside can't escape and stays trapped and builds up inside the object. When the timing is off, incoming energy runs against the object's opposite motion and cancels out. When the timing matches, each new push simply adds to the energy already there. Of course, friction and air resistance drain energy little by little, so the swinging doesn't grow forever. It stops at some size.
This is why resonance can sometimes cause huge disasters. If a regular force, like the marching beat of soldiers' footsteps, matches a bridge's natural frequency, the bridge can sway wildly and even collapse. The 1940 Tacoma Narrows Bridge collapse in the United States is often presented as an example of resonance. But it is explained as "flutter": the wind did not push at a steady beat, and instead the bridge's own motion interacted with the wind and made itself grow.
That's why designers of skyscrapers and long bridges calculate natural frequencies, and some buildings have a heavy weight at the top, a damper (a vibration-absorbing device), to cut down swaying. These devices exist to prevent destruction caused by resonance.
Common misconceptions
The harder you push, the bigger the vibration always gets, so resonance simply happens when you apply an enormous force.
The key to resonance isn't the size of the force. It's the timing (the frequency). With the right rhythm, the same amount of force can create a much bigger swing. Still, friction and air resistance drain energy little by little, so vibration doesn't grow forever, which is why it takes a pretty loud sound to break a wine glass.
Where you meet it
A soprano singer holds a note that matches a wine glass's natural frequency and shatters the glass.
The empty space inside a guitar body resonates with the string's vibration and fills the whole room with sound.
In one sentence
Resonance is when a force applied at the same rhythm as an object's natural frequency makes its vibration grow sharply.