Question

Which part of the wave is changed when there is interference? the amplitude the crest the trough the wavelength

Answer 1

Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium. To begin our exploration of wave interference, consider two pulses of the same amplitude traveling in different directions along the same medium. Let's suppose that each displaced upward 1 unit at its crest and has the shape of a sine wave. As the sine pulses move towards each other, there will eventually be a moment in time when they are completely overlapped. 

Answer 2

The amplitude.

The reason why is because an interference is when two waves combine to form a wave with a smaller amplitude than either original wave. It occurs when the crest of one wave overlaps the trough of another wave. If the crest has a larger amplitude than the trough of the other wave, the crest "wins" and part of it remains.