caption: The image shows a white background with a diagram of a line with a point at the end of it and a line going through it, along with some text written on it. The text reads "2d cos a s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19".