If 2 wave trains, launched at the same moment in time, travel the same distance to reach the same point, they will arrive

1
If 2 wave trains, launched at the same moment in
time, travel the same distance to reach the same
point, they will arrive
in phase and interfere constructively
If one has farther to travel, it will arrive a
bit late (out of phase)
2
P
If one wave train has farther to travel one-half
wavelength further to get to point P, the two
waves will arrive

• in phase 2) 45o out of phase
  3) 90o out of phase
• 4) 180o out of phase 5) 270o out of phase
  6) 360o out of phase

and interfere
1) constructively 2) destructively
3
By the time the wavefront from the right arrives
at point P, the wavefront from the left has
already reached and passed point P, extending ½
wavelength beyond. Its 1st trough will overlap
with the other waves crest. Its crest follows,
meeting the other waves trough. Each portion of
the wave from the right will always be ½
wavelength out of phase with the waves from
the leftthis will continue to be a node.
4) 180o out of phase
2) destructively
A path length difference of ½ wavelength means
the Waves that meet at P will always be 180o out
of phase.