Young's double slit experiment presentation

1. Apr 29, 2007

Deus_Ex_Machina

I am doing a presentation for my high school physics class on light. I want to show the wave-particle duality of light, but first I want to show each separately. So I want to introduce Young's Double Slit Experiment to show the wave nature of light.

The problem is... I apparently don't know how to set up this experiment because I only get two bands of light instead of the multiple bands I ought to get.

I would greatly appreciate if anyone could tell me how to set up this experiment to get the classic result.

2. Apr 29, 2007

lpfr

It is very difficult to do the experience with two real bands. It is easier with a Fresnel bi-prism. The problem is to have the light of two slits coherent and to have enough light to see something. The best way to show interference patterns is to use the eye as a camera. The eyes forms in the retina the diffraction image that you see at infinity and the retina is very sensitive.
What are your means? What do you have as light source and as slots? How many students must see it? It is very hard to project a visible image on a screen. It is easier to have two slots very near the cornea and look at a point source of monochromatic light, a pointer laser reflecting on a very small reflecting ball of ball bearings.

I am curious to know how you plan to show the duality wave-particle. To do it in a classroom is a performance.

Last edited: Apr 29, 2007
3. Apr 29, 2007

Staff: Mentor

You didn't tell us anything about your setup, so it's kind of hard to suggest improvements.

Starting from scratch, a laser pointer makes a good coherent light source. For the slits, try cutting into a piece of aluminum foil with a razor blade. Don't try to cut out a rectangular opening for each slit, just pierce the foil so each slit is a fraction of a millimeter wide, and no more than one or two millimeters apart. Both slits have to fit inside the width of the beam.

4. Apr 29, 2007

cesiumfrog

Quickest way is with a laser pointer, a white wall in a dim room, and two hairs from your head.

5. Apr 29, 2007

DeepQ

Some laser pointers already come with little attachments that screw onto the end and produce the interference patterns. If you look at those attachments closely, you'll see variations of the "slit" method used to do it.

6. Apr 29, 2007

Deus_Ex_Machina

Thank you guys, I was looking for the simplest set-up. It's getting late and presentation is tomorrow, so I'll update after that.

7. Apr 30, 2007

lpfr

Even if now it is too late, I will tell you what I guessed about your problem.

Your slits are too large. If you want that the light from the two slits interfere, the light exiting the slits must "fan out" in order to superpose on the screen.
Let's make a short estimation. Suppose that you want 1 cm of interference fringes at 1 meter of the slits. It means that the diffraction image of each slit must be 1 cm wide at the screen level. If lambda is the wavelength, and w the slit width:
$${\lambda\over w}={1\,cm\over 1\,m}}$$
if $$\lambda=600\,nm$$ you get $$w=60\,\mu m$$. This is much less than an hair width.
Of course, to have 10 fringes 1 mm wide, the two slits must be $$600\,\mu m$$ apart.

The problem is that there is no much light that passes through such thin slits.

We can talk later when you give more details about what you want (and what you can).

8. May 1, 2007

StarChild47

Hi, I'm Laura. I would've put this thread under the homework forum, but since it's already here, I'll just reply.

I'm also coming across a similar problem. I'm doing a science project, and I wanted to show the diffraction of light, and a introduction to wave-particle duality.

I don't need to do it as a presentation, but I wanted to shine a laser through two slits, onto photographic negative paper, therefore taking a 'photo' of the diffraction pattern, so as long as the light is visible, even if very small, it would work fine.
The project is due on Friday, so I have a little time, but I wanted to get it working today, as it's a public holiday.

I tried the aluminium foil, with a scalpel blade actually, and I can get two slits with a laser pointer against a white background in a dark room. I think the problem comes in with the equation above. Obviously i can't get the two slits too close or too narrow, as I'm using the a laser pointer, aluminium foil and a blade. So how far does the distance between slits and the surface I'm projecting onto have to be to make it visible?

Something that's been confusing me too: Isn't a laser pointer designed to not diffract too much? (compare laser pointer and torch at a distance) To make the light travel pretty much straight? Could it pass straight through the slits and not diffract around the edges and interfere?

Thanks! :)

9. May 1, 2007

lpfr

Laser pointers light is the same laser light as others cheap lasers.
You cannot design a light to do not diffract.
Torch light also diffracts, but you cannot see it because it is not monochromatic and coherent light. And the source is not point-like and the optics is of the worst quality.

I do not think that you can do Young slits by hand.
But you can show diffraction images if you take a fine weaved tissue. Try all transparent tissues that you have at hand: silk scarves, semi-transparent drapes, stockings, pantyhose's, etc. The best tissue is not at hand, it is the tissue used for the silk screen printing. It is a thin very small square mesh tissue. To test it, you do not need to have the laser with you, just look through the tissue to a far, intense point light as a far streetlight. You will see the diffraction pattern of a square mesh.
Another beautiful diffraction image can be made with a small hole. It must be small enough, but it is feasible.
Put an aluminium foil between to paper sheets. Hold it thigh over a hard surface (glass is good) and puncture it with a very thin and sharp needle trying not to move laterally the needle. Make several holes. Most of them will not be round.
And try. If the holes are too large, use a thinner bottom paper.
You can also obtain magnificent diffraction images with the light reflected by the good face of a CD.

10. May 1, 2007

StarChild47

Thanks,
In regard to the first answer, i think I get it! I think I wasn't clear as I was explaining my question too, but I understand.

I actually got the experiment to work, with a hole, using this site:Thomas Young's Experiment with a laser pointer, a pin-hole, a piece of card, a lot of prestick and about half an hour of patient alignment. The pin-hole has to be really tiny, but it works, I got a diffraction pattern. (It's also really small, but you can see it!)
I'll try the CD at some point too, even if just for fun
I'm working on my write-up now, but thanks so much!

11. May 1, 2007

lpfr

Congratulations! I read the site and I found that it is a complicated way to do things. If instead "a slip of card" you use a bit of copper wire from a multithread copper wire it easier to adjust. And the width of "slits" is hard to master. Anyhow if you succeeded it is very well.

Diffraction with CD does not give fringes. They behave as diffraction gratings and you can decompose light better than with a prim.

12. May 1, 2007

DeepQ

In the past, I've just used film. You photograph (with B/W film) a few dark lines on white paper, then use the negative (clear lines on dark) for the slits. It works quite well and gives you precise control over the spacing and size of the slits. You can easily do multiple slits, other patterns... endless fun.

It may also be possible to use those plastic sheets (overhead projection sheets) and a laser printer. I've not tried it, but it would be really easy to confirm that it works or not.

And, the other fun thing... you don't need the laser. If you hold the film close to your eye, and use a distant monochrome light source like a street-light, you can get all kinds of strange patterns. You'll have to defocus your eye. Works nicely. Carry them around with you and try different light sources. (Plus, your friends will be impressed.)

Then finally, for the most fun on a Saturday night: if you are nearsighted, remove your glasses/contacts, stand back from a window screen 2-3M and look at a distant streetlight. Adjust your eye focus. Look closely. Give it time. You will be amazed at what you can see.