After displaying three diagrams showing 1, 1 1/2, and 2 wavelengths, the slide show presents more diagrams and expects pupils to determine the number of wavelengths in each. No labels or explanations are provided, so this would be useful after a lesson on the topic. Sometimes I find this mathematical approach to graph interpretation is useful before any information about the context of waves, because the students are not distracted trying to translate the diagrams into wave behavior.