Move the slider to shift one wave relative to one another by a fraction of a wavelength. The red wave is the superposition of the two waves.

When there is no shift of one wave relative to the other, the waves add constructively.[br]If the waves are shifted relative to one another by half a wavelength, the waves add destructively (i.e. the waves cancel one another)[br]If the waves are shifted relative to one another by a full wavelength, the waves add constructively again.[br][br]What happens if the waves have a relative shift of 1.5 wavelengths? 2 wavelengths?[br][br]The shift of one wave relative to the other may be the result of one wave travelling a longer path than the other in reaching the point in space where they interfere, as in the case of Young's double slit experiment.