An animation showing the propagation of a longitudinal wave as multiple black dots apparently moving from left to right. When looked at closely, however, individual dots are repeatedly moving backwards and forwards within a small horizontal window (about 5 millimetres in this diagram). Compressions occur when the dots coincide vertically and rarefactions occur when the dots are less well aligned. Compressions and rarefactions genuinely move across the screen from left to right, even though the particles within them do not. One compression has a black up arrow labelled ‘wave’ below it that follows it across the screen from left to the centre. Five individual dots have been coloured red to make it easier to see their individual back-and-forwards motion, and two of these dots are labelled with a red up arrow and the word ‘particle’ to exaggerate the motion further.