Which term describes a wave at the boundary between density layers within the ocean?

• A. Capillary waves
• B. Whitecaps
• C. Wind shadow definition
• D. Internal wave

Answer: (D)

Internal waves form at the boundary between layers of different density inside the ocean, where a sharp density change (the pycnocline) separates a lighter layer from a heavier one. The gravity pulling on that density difference acts as the restoring force when the interface is displaced, so the wave travels along that boundary rather than on the surface. This distinguishes them from surface phenomena like capillary waves, which are tiny surface ripples governed by surface tension, or whitecaps, which are surface features created by wind breaking the crest. The option described by “a wave at the boundary between density layers” is the internal wave, which can influence currents and water movement below the surface—an important consideration for paddlers who might encounter abrupt flow changes even if the surface looks calm.