When two ball-ended dippers are attached to the vibrator of the ripple tank, two sets of circular ripples are sent out which pass through one another as shown in Fig. 26.10. Where the two waves are superposed in the same phase, e.g., crest on crest, we get
lines of increased disturbance or constructive interference. These are called the antinodallines. In between these are the nodal lines along which the waves are exactly out of phase, e.g., the crests of one are superposed on the troughs of the other. Here,
provided the amplitudes of the two waves are the same, we now get zero resultant disturbance of the water surface, or destructive interference.
A similar interference pattern is obtained if either a straight or a circular wave is incident on a vertical barrier having two small apertures. In this case, interference takes place between the emerging diffracted waves (see Fig. 26.11).