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# Long description

There are three uniformly spaced horizontal dotted lines going across the diagram. Each such line has five red dots uniformly spaced along it, so that the dots form five columns each with three dots in it. The vertical distance between adjacent horizontal lines is shown as 'd'.

Three parallel red arrows are shown coming from the upper left and heading down and to the right. The top arrow is labelled 1 and the arrow below it is labelled 2. Arrow number 1 is heading for the third dot on the top horizontal line. Arrow number 2 passes through the first dot on the top line and heads for the third dot on the second line. The angle between arrow number 1 and the top horizontal line is shown as 'theta'.

A vertical line has been drawn down through the third red dots on each line, and a line has been drawn through the third dot on the top horizontal line, down and to the left, so that it crosses arrow number 2 at right angles. The point where it crosses arrow number 2 is labelled 'A', and the third dot on the second row is labelled 'B'.

The right hand side of the diagram is a mirror image of the left side, as if it had been reflected in the vertical line which was drawn through the third red dots on each horizontal line. The equivalent point to point 'A' on the left hand side has been labelled 'C'.