Which of the following curve best represents the frequency received by observer $f$ as a function of time?

Which of the following curve best represents the frequency received by observer $f$ as a function of time? Four graphs showing frequency $f$ versus time $t$ with different step and curved variations.
  1. Graph (a)
  2. Graph (b)
  3. Graph (c)
  4. Graph (d)

Solution

While approaching, $f > f_0$ but $f = \text{constant}$. While receding, $f < f_0$ but $f = \text{constant}$.

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