# Homework Help: Double slits

1. Sep 9, 2011

### v_pino

1. The problem statement, all variables and given/known data
Find the de Broglie wavelength for an electron with v=0.001c. Find the angular width of the central bright fringe in a double slit experiment, with the separation of the two slits d=50nm.

2. Relevant equations

wavelength = h/mv

d sin(theta)=n*wavelength

3. The attempt at a solution

For the wavelength, I got 2.42x10^-9m. I think this sounds correct.

Is my second equation for diffraction applicable here? I know that it is double slits so there will be interference and not just diffraction. If this equation is applicable, the my answer is 2*theta = 5.55 degrees.

Or is there a specific equation for double slit that I should use?

2. Sep 9, 2011

### vela

Staff Emeritus
How can you calculate anything about diffraction without knowing the slit size?

3. Sep 9, 2011

### v_pino

So does it mean that the question is just asking for the angle to the first maximum as in that of a diffraction pattern using dsin(theta) = n*wavelength?

4. Sep 9, 2011

### vela

Staff Emeritus
Not exactly. You want to find the angle between the first-order minima, since they border the central maximum.

5. Sep 9, 2011

### v_pino

Sorry for not understanding the simplest of the idea here. So are you saying that the diffraction equation can be used in the question even though it is asking for interference? If so, the answer should simply be 2*theta with theta being from the equation d*sin(theta)=n*wavelength?

6. Sep 9, 2011

### vela

Staff Emeritus
Where did I say to use the diffraction equation?

7. Sep 9, 2011

### v_pino

I don't get where you are trying to lead me here. Am I missing a big clue? Should I be using some sort of approximations? Plus I don't know the distance from screen to source - should I be estimating this?

8. Sep 9, 2011

### vela

Staff Emeritus
What does "angular width of the central bright fringe" mean?