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Stationary (or Standing) Waves
If two sets of identical waves travel in opposite sense though the same medium, interference occurs between them.
The interference can be constructive or destructive.  
Places where destructive interference occurs are called nodes or nodal points (or lines).  
Places where constructive interference occurs are called anti-nodes or anti-nodal points (or lines).  
The pattern of nodes and anti-nodes remains fixed in space and is called a stationary (or standing) wave*.  
See here for animation illustrating the idea.  
 
In an unbounded medium (consider ripples on the surface of water, a long way from the edge of the pool) stationary waves of any wavelength can be established by adjusting the frequency of the oscillators producing the waves.  
   
Of particular interest are standing waves produced by reflection of waves at boundaries.  
For example, when a string under tension is disturbed at a point, waves travel away from the point where the disturbance occurred.  
The waves are then reflected at the fixed ends of the wire.   
Therefore, we have two waves of the same frequency travelling in opposite sense along the wire.  
However, the ends of the string are fixed in place so this means a node exists at each end of the string.  
   
For this reason, you can only have a stationary wave on the string for certain frequencies of oscillation, corresponding to certain wavelengths which "fit into" the length of the string.  
When the string is caused to oscillate at one of these special frequencies, it responds with a large amplitude oscillation and is said to be resonating (or in resonance).  
   
Similar resonance situations can occur in other oscillating systems, for example, a mass on a spring, a pendulum, air columns in musical instruments, bridges and other structures, car engines, electric circuits... the list is virtually endless!  
 
* A rather confusing term since a wave, by definition, moves.  
However, I suppose itís easier to say "stationary wave" than "stationary distribution of nodes and anti-nodes produced by destructive and constructive interference between similar waves moving in opposite sense through a given medium"!  
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