Question

The figure shows a sound wave as a function of time when the source is stationary with respect to the observer.

An Illustration of a transverse wave and four crests and four troughs are labeled 1 s total.
Which shows the perceived sound wave heard by a stationary observer when the source moves toward the observer?

An illustration of a transverse wave and three crests and three troughs are labeled 1 s total.
An Illustration of a transverse wave that is taller than the previous. Three crests and three troughs are labeled 1 s total.
An Illustration of a transverse wave and four crests and four troughs are labeled 1 s total.
An Illustration of a transverse wave and seven crests and seven troughs are labeled 1 s total.

Answer 1

The one showing seven crests and seven troughs in 1 s.

Explanation: motion of the source toward the observer compresses the wavefronts (shorter wavelength) so the observer detects a higher frequency (more cycles per second).