I don't know if this would be considered a homework problem, so I hope I'm posting in the right place. I'm working through an independent learning course and just needs some help with the terminology in the book. It started expressing wavelengths in 1/2λ, and I can't find an explanation why. I've come to this point in the text: http://img542.imageshack.us/img542/1317/ovlt.jpg [Broken] It states: "Since each circle in Figure 13.25 corresponds to a wave, the distance between each circle is one wavelength, or 1 1/2λ. Therefore, the distance from the Source S1 to the point P1 on the first nodal line is 3λ. Similarly, the distance between S2 and P1 is 2.5 1/2λ. Therefore, the difference in the distance between the line segments P1S1 and P1S2 is 0.5 1/2λ." I assumed each circle is a crest in the wave, so a full wavelength. Are they actually crests and troughs alternating to make them half wavelengths? But then why is the distance from S1 to P1 3λ and not expressed as 1/2λ? Also, the part where it states "the distance between each circle is one wavelength, or 1 1/2λ" seems to be contradictory. Is it one wavelength or one half wavelength? Any help understanding this would be greatly appreciated!