Reading for Class 7: Interference and Diffraction of Light Waves
Interfering Waves  Two traveling waves can interfere with each
other.  You may remember how to add sine waves from a trig class
long ago.  You merely add the functions at each individual
point.  For example, consider the three cases illustrated below.
In case (a), the two waves match up exactly.  We say they are
"in-phase", since everywhere Wave 1 has a crest Wave 2 also has
a crest, and everywhere Wave 1 has a trough, Wave 2 also has a
trough, etc.  When these two waves add, the result is a sine
wave with twice the amplitude as one of the original waves.  The
waves in this case exhibit complete constructive interference.
In case (b), the waves are "180º out of phase", which means that
they are off by one-half cycle.  Every crest in Wave 1 matches a
trough in Wave 2, and vice-versa.  Adding these two waves yields
an amplitude of zero.  This case illustrates complete
destructive interference.  Case (c) illustrates a situation
somewhere betwee