Probing questions
2. Let's use the term frequency of waves perceived by the observer to describe the frequency at
which waves are passing through the receiver. How do you describe the frequency of waves
perceived by the observer from the moment the source appears to the moment it disappears?

the frequency increases and decreases.

Explanation: According to doppler effect, the frequency of waves tends to increase as the source approaches and decrease as the source disappears.

The Doppler effect occurs when a source of waves and/or observer move relative to each other, resulting in the  observer measuring a different frequency of the waves than the frequency that the source is emitting.