I am studying low and high pass RL filters for Navy schooling. I need someone explain why the frequency cutoff is $fco = \frac{R}{2\pi L}$ mathematically.

From my oscilloscope lab results I got the following. $V_{pp}$ represents the peak to peak voltage :

\begin{array}{|c|c|} \hline kHz & V_{pp} \\ \hline 0.1 & 12 \\ \hline 1 &12 \\ \hline 10 &11.73 \\ \hline 100 & 5.17 \\ \hline \end{array}

The lab asked us to to find the value of $0.707 \times 12$ to determine where the change over occurred. Conceptually, I understand that the root mean square method is used to determine AC's efficiency in comparison to DC. AC are about 70.7% as effective as DC.