\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\)

Finding Wavelength

You can measure the wavelength from the snapshot graph: it is the distance between two successive crests, troughs, or other matching parts on the wave. Alternatively, you can count the number of cycles $N$ that occur during a certain distance $d$, and then $\lambda=d/N$.

If you are given a history graph and the wavespeed, you can measure the >waves/period period from the history graph, and then use the formula $v={\lambda\over T}$.