Those of you who are physicists, I hear you rolling your eyes, but bear with me.  To measure the wavelength, all I need to do is measure the distance from here -- one full wave -- over to here.
物理の専門家が聞いていたら目を回すでしょうが ご勘弁ください  波長を測るには ここからここまでの距離を 1つの波の長さを測ればいいのです