# Double Slit Experiment Project

1. Aug 14, 2012

### DLHill

For my AP Physics course, I am trying to replicate the double slit experiment. My idea is to have a cathode ray tube from an old television set and take the electron gun out. The electron gun will be firing at a plastic or foil sheet with 2 narrow slits side-by-side. Behind the screen, I will have a scintillator to show where the electrons hit.
Will this setup work? What could I change to make it better?
Thank you

2. Aug 14, 2012

### Simon Bridge

In order to get diffraction, you need the separation of the slits to be comparable to the wavelength in question. What is the deBroglie wavelength of the electrons from the gun?

3. Aug 14, 2012

### Staff: Mentor

Do you have a vacuum container and pump and all that? You can't just fire electrons through the air as they will impact the air molecules nearly instantly.

4. Aug 14, 2012

### DLHill

The entire thing will be in an aquarium which will be turned into a vacuum chamber

5. Aug 14, 2012

### DLHill

I'm not entirely sure about that. I am going to try and use a tungsten filament possibly instead of the cathode ray tube. How would i find the wavelength of the electrons if I used a filament?

6. Aug 15, 2012

### Simon Bridge

About what? That you need very narrow slits for diffraction or what the deBroglie wavelength of an electron is?

See how other people do it:
http://www.chem.qmul.ac.uk/surfaces/scc/scat6_2.htm [Broken]
... notice the grating is a crystal? This gives angstrom-spaced slits.

$\lambda_{dB}=h/p$
... you want low speed electrons, but you also need to accelerate them to your slits in something like a beam. Remember that the angle of diffraction maxima is predicted to be $d\sin\theta = n\lambda_{dB}$ you want $\Delta\theta$ to be big so you can detect it.

Electrons thermally ejected from a solid have energy $\frac{3}{2}kT - \phi$ (? check) where $\phi$ is the material's work function. For tungsten that is about 4.5eV (?) But you will be accelerating them through a voltage to get the beam.

There are other electron sources - you can get good effects with a Van der Graaf generator for eg.

Last edited by a moderator: May 6, 2017
7. Aug 15, 2012

### DLHill

I just didn't know how to find the deBroglie wavelength. I'm going to read all of this and figure it out.

8. Aug 15, 2012

### ZapperZ

Staff Emeritus
You need a potential gradient.

The cathode tube that you took the gun out has a grid that has been applied a positive bias. This provides the potential gradient that accelerates the electrons to a particular energy/velocity. If you don't have this, then the electrons that is produced out of the filament will go everywhere, or in a direction that isn't where you want them to go.

The energy/velocity gained by these electrons will give you an estimate of the deBroglie wavelength, which will then tell you the order of the size of the slit.

Zz.

9. Aug 16, 2012

take a two tranperent glass sheets and laser light, make the glass sheet black nontransperent with lamp by spreding smoke on it. take a pin and make one small slit on one glass and two slits on another glass. now place the glass sheets one after other as single slit and double slit. then pass a laser light in it. you can get a interference fringes on the screen.

10. Aug 16, 2012

### Simon Bridge

Welcome to PF madhuvenkat
- perhaps OP explicitly wants to see electron diffraction?

BTW: A trick to make slits low-tech-wise is to use two razor-blades flat to each other. That way you get the slit separation = width of one blade, and they end up parallel.*

If you use a laser - you don't usually need the initial single-slit.
If you don't have a laser, you can put a small light source at the focal point of a bi-convex lens.

----------------------
* Can you even get disposable razor blades any more? Maybe a pair of scalpel blades? I know - you can draw two lines on white ppaer and photograph it - then, then you get the negative ... what do you mean "nobody uses film any more" sheesh!

11. Aug 16, 2012

### sophiecentaur

If you intend to dismantle an old TV tube then you should be aware that the phosphors on the tube face contain some toxic elements (e.g. cadmium). Also, you need to be careful when letting down the vacuum because it is possible to create an uncontrolled implosion and for the flying glass etc. to injure you.
Also, the classic 'double slit' experiment was really based on light waves. The best way to demonstrate electron diffraction is to use a thin carbon target, rather than two slots. Unless you have some fancy manufacturing facility, you will find this difficult. Doing it with light (to prove a point to yourself) would be more fruitful and easy to achieve - at least at first.

12. Aug 16, 2012

### DLHill

Thank you for this information. I will probably start with a basic setup using a laser now.

13. Aug 16, 2012

### sophiecentaur

Very sensible of you. It will still be VERY satisfying for you and you can do the sums to connect wavelength, fringe separation etc. much easier with light as you will know the wavelength with more certainty.