This immense wall of moving water gives the surfer an amazing ride. The swelling surf raises him up and pushes him forward as though he’s as light as a feather. All he needs to do is keep his balance on the surfboard. The incredible power of the wave will do the rest. Ocean waves like this one are examples of surface waves.
Waves on the Surface
A surface wave is a wave that travels along the surface of a medium. The medium is the matter through which the wave travels. Ocean waves are the best-known examples of surface waves. They travel on the surface of the water between the ocean and the air.
Q: What do you think causes ocean waves?
A: Most ocean waves are caused by wind blowing across the water. Moving air molecules transfer some of their energy to molecules of ocean water. The energy travels across the surface of the water in waves. The stronger the winds are blowing, the larger the waves are and the more energy they have.
How the Medium Moves
A surface wave is a combination of a transverse wave and a longitudinal wave. A transverse wave is a wave in which particles of the medium move up and down perpendicular to the direction of the wave. A longitudinal wave is a wave in which particles of the medium move parallel to the direction of the wave. In a surface wave, particles of the medium move up and down as well as back and forth. This gives them an overall circular motion. You can see how the particles move in the Figure below.
Why Waves Break
In deep water, particles of water just move in circles. They don’t actually move closer to shore with the energy of the waves. However, near the shore where the water is shallow, the waves behave differently. Look at the Figure below. You can see how the waves start to drag on the bottom in shallow water. This creates friction that slows down the bottoms of the waves, while the tops of the waves keep moving at the same speed. The difference in speed causes the waves to get steeper until they topple over and break. The crashing waves carry water onto the shore as surf.
Q: In this diagram of a wave breaking near shore, where do you think a surfer would try to catch the wave?
A: The surfer would try to catch the wave where it starts to steepen and lean forward toward the shore.
- A surface wave is a wave that travels along the surface of a medium. Ocean waves are the best-known examples of surface waves.
- In a surface wave, particles of the medium move up and down as well as back and forth in an overall circular motion. The particles don’t actually move closer to shore as the waves pass through.
- In shallow water close to shore, waves start to drag on the bottom of the water. The bottoms—but not the tops—of the wave slow down because of friction. This causes the waves to steepen until they break and fall over, carrying water onto the shore as surf.
- What is a surface wave?
- Describe how particles of the medium move in a surface wave.
- Explain how surface waves change when they reach shallow water near shore.