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Here are two "ripple waves" like the kind you would see if you dropped a pebble into a still pond:
The wave on the left has a little cone sticking up into the air. The wave on the right has volcano-like dent in the middle of it.
You may be suprised to learn that these two pictures are identical except for a 180 degree rotation. Almost everyone who looks at these pictures sees a bump in the one on the left, and a dent on the one on the right.
This is probably caused by our ordinary experience with lighting conditions. Generally speaking, we see things lit from above (by the sun, or an overhead light). If we assume that the light is above us, we sould expect bumps to appear bright on the front side and dark on the back side, and vice versa for dents. This leads us to the natural assumption that the left figure has a bump, and the right figure has a dent.