1. The problem statement, all variables and given/known data Light of 650 nm can be seen in three orders in a particular grating. About how many slits per cm does this grating have? 2. Relevant equations Yikes. 3. The attempt at a solution Either this question is ridiculously hard resulting in me not even having a clue where to start, or it is so easy that I don't even deserve to get it right.. haha. If someone could just point me towards the right CONTINENT, I might have a chance.. heh. Thanks in advance.