There are two dots at the bottom of part (a). One dot is a distance 'd' to the right of the other. A vertical arrow points straight down to each dot. These two arrows are labelled 'incident beam'. On the left hand arrow there are two and a half cycles of a wave shape, and on the right hand arrow one and a half cycles of the same wave shape.

Going up and to the right from each of the dots are two more parallel arrows. The first has three cycles of the wave shape along it, the second has only two cycles of the same wave shape. The two arrows are then merged to form a single arrow pointing in the same direction. This arrow has three and a half cycles of the wave shape along its length.

Part (b) is very similar, but the two vertical arrows both have two and a half cycles of the wave shape along them. The two arrows which go up and to the right are heading in a slightly different direction, and the first has five cycles of the wave shape along it, while the other has three and a half cycles of the wave shape along it. The two arrows are then merged together to form a single arrow pointing in the same direction, but there are no wave shapes drawn along this arrow.