1. The problem statement, all variables and given/known data The same diffraction grating is used with two different wavelengths of light, λA and λB. The fourth-order principal maximum of light A exactly overlaps the third-order principal maximum of light B. Find the ratio λA/λB. 2. Relevant equations sin theta = m lambda / width of the slits 3. The attempt at a solution sin theta 1 = sin theta 2 since they overlap. 3/4 I don't understand how can 4th order principal and 3rd order principle can have the same sin theta? Because, unless I am misunderstanding something, I don't think it's possible for different orders to completely overlap on top of each other so that they meet at the same point on the opposite screen of the diffraction grating.