Figure 2-5 An Experiment with Water Waves The intensity of light on the screen shown in Figure 2-4 is analogous to the height of water waves that pass through a barrier with two openings. (The illustration shows this experiment with water waves in a small tank.) In certain locations, wave crests from both openings reinforce each other to produce extra high waves. At other locations a crest from one opening meets a trough from the other. The crest and trough cancel each other, producing still water.