define superposition of light waves in context to diffracation of light throgh a single slit
When two or more wave motions traveling through a medium superimpose one another, a new wave is formed whose resultant displacement at any instant is equal to the vector sum of the displacement due to individual waves at that instant
The two short lined waves traveling in opposite direction first add up (center) to form a resultant wave and then move off as if nothing happened to them.
(It is important to note that each wave preserves its individual characteristic of wave motion)
This shows that when an up-wave (crest) meets a down-wave and are identical, then the resultant is a straight line.
When two down waves superimpose one another, a new wave is formed but in a downward direction (the amplitude of the resultant wave is the sum of the amplitudes of two waves).
Hope this helps.