# Homework Help: Velocity of a Helium-atom in a double slit experiment

1. Jul 29, 2016

### laguna

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.

2. Jul 29, 2016

### BvU

Hello Laguna,

Apparently you need one or more relevant equations to proceed. SInce there are none under '2. relevant equations', you will now have to search your toolbox to find some

Also: in your attempt at solution you might show your steps in more detail; if something goes wrong, PF helper can put it right....

3. Jul 29, 2016

### composyte

Your difficulty might come from not having all the relevant equations. Think about how the velocity of a particle is related to its wavelength via an important equation.

4. Jul 30, 2016

### laguna

thank you for the warm welcome :-)

I can use the de-Broglie relation lamba=h/p, p the momentum. then i get the speed, right? :-)
I am sorry, I missread '2. relevant equations' with '2. relevant questions' when I was writing the post.

5. Jul 30, 2016

### BvU

Good start. You want to check if you need anything relativistic and then you can link $\lambda$ and $p$ (or fist link them and then check that $v<<c$)

6. Jul 31, 2016

### laguna

thank you. i didnt even condsider relativity.