1. The problem statement, all variables and given/known data The line spectrum of a certain substance consists of three prominent lines; blue (B), yellow (Y) and red (R). When the spectrum is examined with a diffraction grating having d = 4 * 10-6 m, it is found that the sequence of lines, moving from the centre, is B, Y, R, B, Y, B, R. Further, it is found that the diffraction angles θ of the fifth and seventh lines are 17.46° and 20.49° respectively, and that the sixth line is at an angular position exactly halfway between them. Find the wavelength of the blue line. Answer: 4.34 * 10-7 m 2. The attempt at a solution We find the θ6 = (17.46 + 20.49) / 2 = 18.975°. Then plug into the formula: n λ = d sin θ to find the wavelength of the sixth line: λ6 = (4 * 10-6 * sin 18.975°) / 6 = 2.168 * 10-7 m. I also found the wavelengths for the fifth and seventh lines: 2.4 * 10-7 m and 2 * 10-7 m respectively. But after this I don't know what to do. I need to find the wavelength of the first blue line, while I have the wavelengths of a yellow (5), blue (6) and a red (7) one. How to use them to find blue (1)?