Final answer:
Two waves exactly out of phase result in pure destructive interference, canceling each other out completely, while waves in phase lead to pure constructive interference, doubling the amplitude without altering the wavelength.
Step-by-step explanation:
When two waves arrive at the same spot exactly out of phase, such as a crest meeting a trough, pure destructive interference occurs. According to the principle of superposition, these waves exactly cancel each other out because their disturbances are in opposite directions, resulting in a resulting amplitude of zero. This means that at the point where they meet, there will be no resultant wave, effectively creating a flat line or no disturbance at all.
On the other hand, when waves are perfectly in phase (crest meets crest and trough meets trough), they will interfere constructively. For identical waves, this pure constructive interference doubles the amplitude of the individual waves without changing the wavelength, creating a wave that is twice as tall as the original waves.