Have you ever noticed seagulls bobbing up and down on the ocean? You might have also seen surfers catch a wave that takes them to shore. Maybe you have floated on a lake, going up and down as a wave passed by. Or perhaps you have seen debris, such as driftwood, that has been washed up by waves. Water waves are fascinating—they come in all sizes, from a tiny ripple to monster waves that are 10 meters high. You have seen them, but do you know what drives them—and how they move across oceans and seas?
In this activity you will bring the ocean home and make waves in a bottle. You will also sharpen your observation skills and find out why some waves are slow, and others are fast.
Wind causes much of the waves on seas and oceans. Here's how it works: Wind causes water to pile up above the undisturbed water level. The water under the crest (or peak) of a wave gets the extra water pushing down on it. The extra weight then pushes water out from under the peak of the wave. The moving water overshoots and creates a peak in a new place, and so the wave moves on.
Oceans and seas are layers of water under a layer of air, the atmosphere. Because air is much less dense than water (weighing about 1,000 times less than the same quantity of water), it seems like air would have little influence on a wave. So in order to understand how waves work, you will add a layer of oil on top of the water. Curious about how this relates to air and affects a wave on the water surface? Roll up your sleeves and try this rocking science activity!
Did you notice that water with oil on top produces surprisingly slow waves?
A wave moves because the extra weight of the liquid in the wave's peak pushes water from under the peak to places where the water is shallower. This happens in both bottles.
For the bottle with water and air, both the water and the air push on the deeper layers of water. Air is about one thousand times less dense than water, so when a wave passes by, deeper layers of water mainly get the weight of all the water piled up in the wave. The difference in weight between the crest and the trough (or lowest point) is the wave's driving force. In this scenario, the big difference in weight results in a fast-moving wave.
Similarly, for the bottle with water and oil, water and oil push on the deeper layers of water. Just like air, oil is less dense than water (about two thirds as dense), so an amount of oil weighs less than the same amount of water. Thus, as water piles up in a wave, more water and less oil will weigh down on a spot underneath the wave peak. At the shallowest point of a wave, however, more oil and less water weighs down on the water below. As a result, the difference in weight of the liquids at a point under the crest and a point under the trough (or lowest point of the wave) is much smaller in this scenario, so the driving force of the wave is much smaller, although it still forces water to move from underneath the wave peak. In addition, the water and the heavier-than-air oil both have to be moved, so the liquids flow much more slowly than when the light air flows on top of water. What you observe is a wave on the water surface that travels much more slowly.
In the extra activity you probably observed that the wax mainly moved up and down while the wave moved forward and backward. This is because in a wave it is the water underneath the wave that moves horizontally; the water on the surface moves up and down.
You can pour the contents of the bottle with only colored water down the drain. But do not pour oil down the drain!
Set your bottle with water and oil upright and give the liquids some time to separate. Select a container or bottle (such as the one you used for colored water) in which to collect your oil. Gently pour the layer of oil into a clean container or bottle. It is okay if some water flows over as well, just do your best to transfer most of the oil. (Use a funnel if one is available.) You can pour the remaining colored water down the drain. You can throw your sealed bottle of oil in the trash, compost it or search online for a place to donate your used cooking oil. (Used cooking oil can be turned into biodiesel, a nontoxic and biodegradable fuel that can power cars and buses and produces fewer air pollutants than petroleum-based diesel.)
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.