Observation of which phenomenon can be considered proof that a wave is present ?

This is the number of complete movements of a wave per second.

Sawyer is studying diffraction. he draws a diagram of a plane wave to show how light waves travel. which best describes sawyer’s error? the wave fronts should be perpendicular to the direction in which the waves move. the arrow showing the direction of movement of the waves should be pointing to the left. the arrow showing the direction of movement of the waves should be pointing downward. the wave fronts should be both parallel and perpendicular to the direction in which the waves move.

If the wavelength is tripled, while the speed of the wave is held constant, what happens to the frequency?