1. Fig. 1.1 is a graph of the observed wavelength against time of a sound wave emitted from a single source. The emitted sound wave has a frequency ${f}_{s}$. The sound travels at a constant velocity directly towards an observer and a frequency of ${f}_{T}$ is observed. It then travels directly away from the observer and a frequency of ${f}_{A}$ is observed.

Fig. 1.1

What is the value of the ratio$\frac{{f}_{A}}{{f}_{T}}$?