Consider a transverse wave moving through a medium, for example,
a wave moving through a stretched spring. 

Each point in the medium is oscillating in a direction
perpendicular to the direction of propagation of the wave. 

(If the wave is a sine wave, the oscillation of each point is
the simplest type of oscillation, called simple harmonic motion.) 

The points are all oscillating but different points reach their
maximum displacement at different times. 

To describe this situation we say that a wave consists of many
oscillations which are out of phase with each other. 

The actual phase difference between two oscillations depends on
the distance between them (measured along the direction of
propagation of the wave). 



In many situations, we need to consider two sets of wave, of the
same frequency, travelling through the same medium. 

The combined effect of the two waves at a given point in the
medium will depend on the phase difference between their
oscillations at the point considered. 



We will consider the two extreme cases, that is, 

1. the two waves arrive at the point in phase with each
other and 

2. they arrive in antiphase (p
radians or 180° out of phase) 



1. Two waves arriving at a point in phase result in
constructive interference. 







The resultant of these two oscillations will be an oscillation
of amplitude equal to sum of the amplitudes of the two original
oscillations. 





2. Two waves arriving at a point in antiphase result
is destructive interference. 











In this case the algebraic sum of the oscillations results in an
oscillation of smaller amplitude. 

If the amplitudes of the two oscillations
are equal the result
is no oscillation. 



The fact that the resultant amplitude can be found by adding
together the individual amplitudes is sometimes referred to as the
principle of superposition of waves. 
