Diffraction
Diffraction is caused by one wave of light being shifted by a diffracting object. This shift will cause the wave to have interference with itself.  Interference can be either constructive or destructive.  When interference is constructive, the intensity of the wave will increase.  When interference is destructive, the intensity will decrease, sometimes to a point where it is completely destroyed.  These patterns of interference rely on the size of the diffracting object and the size of the wave.  The strongest examples of diffraction occur in waves where the wavelength is similar to the size of the object causing diffraction.