1. The problem statement, all variables and given/known data Hi, I want to apologize for any grammar errors in advance since english is not my first language. But i hope it is good enough such that the question is clear: I want to calculate the velocity of a Helium-atom after it scattered on a double slit. The following information are given: 1) the distance of the first intensity maximum from the axis of the detector screen 2) distance d between both slits 3) distance from the double slit and the detector screen 2. Relevant equations what is the velocity of the helium-atom? and what velocity would an electron need to have to reach the same seperation of its intensity maximum? 3. The attempt at a solution 1) first I calculated the condition for constructive interference: d*sin(phi)=n*lamba, n integer. 2) from the distance to the screen L, and the distance of the first maximum, say l, we get phi: tan(phi)=l/L 3) from the first step we get lambda, since n=1, right?. And here i am stuck. Thank you.