Ripple Tank Lab: Shallow Water and Spreading Rings

Lab Notes: The Ripple Tank

1Mr. Felix Mbeki-Shaw's eighth grade class used a ripple tank, a square tray 80 cm on each side with a glass bottom and a thin layer of water. Most of the tray held water 2.0 cm deep. On one side, a flat glass plate lay on the bottom, so the water above it was only 0.5 cm deep. Rohan Achterberg and Imelda Sorensen filmed the waves in slow motion with a ruler taped under the glass, so they could measure the waves on the video. Before each test they leveled the tray so the depth stayed the same across each side.

2For the first test, a straight bar on a small motor dipped into the water 4 times each second, sending straight crests across the tank, so the frequency was 4 Hz. Rohan counted the crests passing a mark on the deep side and a mark on the shallow side. Both marks had 4 crests pass each second. Imelda measured the wavelength, the distance from one crest to the next, on each side. Table 1 shows the results.

3For the second test, the class replaced the bar with a small ball that dipped at one point in the deep water at 4 Hz, making circular ripples that spread out in growing rings. Small paper dots floating on the water bobbed up and down as each ring passed, but they stayed in about the same place. On the video, the crests took the same time to travel each 4 cm, near the ball and far from it. When Imelda made the ball dip farther into the water, the ripples were taller, but they still reached the wall of the tank at the same time as before.

4The students measured each ring's amplitude, the height of a crest above the flat water level, at three distances from the ball. Mr. Mbeki-Shaw's wave program used the crest height and speed to work out how much energy each centimeter of crest carried each second, in energy units. Table 2 shows their results and the length of the whole crest circle at each distance. The model leaves out the small amount of energy lost to friction with the tray.