What is defined as the number of waves that pass an observer in one second?

The correct answer is frequency because it specifically refers to the number of complete waves that pass a given point or observer in one second. This concept is fundamental in wave physics and is measured in hertz (Hz), where one hertz equates to one wave cycle per second.

Understanding frequency is crucial in various applications, such as sound and light waves, as it directly correlates to pitch in sound or color in light.

Amplitude refers to the maximum displacement of points on a wave from its rest position, wavelength is the distance between successive crests or troughs of a wave, and speed describes how fast a wave travels through a medium. While all these terms are related to waves, frequency uniquely quantifies the rate at which waves occur over a specific time interval, making it a key characteristic in wave mechanics.