Diffraction is normally taken to refer to various phenomena which
occur when a wave encounters an obstacle. Very similar effects are
observed when there is an alteration in the properties of the medium
in which the wave is travelling, for example a variation in
refractive index for light waves or in acoustic impedance for sound
waves and these can also be referred to as diffraction effects.
Diffraction occurs with all waves, including sound waves, water
waves, and electromagnetic waves such as visible light, x-rays and
radio waves. As physical objects have wave-like properties,
diffraction also occurs with matter and can be studied according to
the principles of quantum mechanics.
While diffraction occurs whenever propagating waves encounter such
changes, its effects are generally most pronounced for waves where
the wavelength is on the order of the size of the diffracting
objects. The complex patterns resulting from the intensity of a
diffracted wave are a result of the superposition, or interference
of different parts of a wave that traveled to the observer by
The formalism of diffraction can also describe the way in which
waves of finite extent propagate in free space. For example, the
expanding profile of a laser beam, the beam shape of a radar antenna
and the field of view of an ultrasonic transducer are all explained
by diffraction theory.