Care must be taken representing waves traveling in opposite directions. Erroneous results can occur when combining waves with the popular convention where a wave traveling to the right is ψR(x,t) = A sin (kxωt) and a left-traveling wave is ψL(x,t) = A sin (kx + ωt). An easily overlooked pitfall is pointed out and a way to avoid it with a secure convention is presented. As an exercise in critical thinking, students can be presented with the pitfall and asked to find a way out.

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