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Diffraction
The phenomena observed when waves are obstructed by obstacles or pass through apertures are called diffraction effects.
All (two or three dimensional) waves exhibit diffraction effects but the magnitude of many of these effects depends on the wavelength of the waves.  
Diffraction effects are most obvious when the object or aperture causing the diffraction is similar in size to the wavelength of the waves.  
   
As with interference, these effects can be seen quite clearly using a ripple tank to observe small waves on the surface of water.  
   
   
If plane waves (the "straight" waves on the left of this diagram) are sent towards a barrier with a well defined edge, they will "bend round" the object.
 
The edge of the barrier appears to behave like a point source of waves.
 
 
   
   
 
 
 
 
Plane waves sent towards a large aperture show the same effect at each edge (duh, of course...)
 
 
 
 
 
However...
 
   
 
 
 
...if the aperture is small enough (of the same order of size as the wavelength, or smaller), it will behave like a point source of waves.
 
 
 
 
 
   
   
 
Plane waves incident on a small object (again, the same order of size as the wavelength, or smaller), it will also behave like a point source of waves but in this case sending the waves back too in what can be considered to be a reflection.
 
This phenomenon is useful to bats which used echo location to find insects.
 
 
An arrow drawn on a diagram to show the direction of propagation of a set of waves is called a ray.  
A ray is always at 90 to the wavefront.  
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