I like the wavelength formula because of it's simplicity and practicality.

  λ = c / f

It is a cool way to demonstrate the importance of the speed of light to figure out the optimal length of an antenna for a given radio frequency.

My son, when asking why knowing the speed of light matters, was impressed when I showed him we could make our own wifi antenna by using this to figure out the length it needed to be. Of course antenna design can be complex but this was still cool to show.