# What are some nice modern physics experiment that I can do?

1. Jun 26, 2012

### elishamei9000

I need some modern physics experiment to be presented on science fair. I have no idea like im thinking of particle physics like atom smashers but it's impossible to create one for just a month. The cornstarch in a stereo has been gotten by others so im thinking of photoelectric i dont know, so please i need suggestions about this. thanks

2. Jun 26, 2012

### A.T.

3. Jun 26, 2012

### QuantumPion

How about a cloud chamber. They are not difficult or expensive to make and would provide a neat visual demonstration of ionizing radiation.

4. Jun 27, 2012

### elishamei9000

yes I was thinking of making a cloud chamber, but I want to see some muons/ positrons. would that cost me much? and is it too long to create one?

5. Jun 27, 2012

### M Quack

I saw an amazing demo a few months ago.

It consisted of an upright transparent tube filled with sand. At the bottom there was an inlet for compressed air that could "bubble up" through the sand, loosening it up and essentially liquifying it.

The demonstrator then took a steel ball and dropped it into the sand. It dropped right through to the bottom, you could hear it hitting the bottom of the chamber. Then he turned the air off and tapped the side of the tube a few times to make the sand settle.
He then dropped an identical steel ball which just landed on the surface of the sand, not through it. Turn on the air, and boom it goes straight to the bottom. He also had a magnet on a string to fish the balls back out of the tube.

The tube was about 25cm in diameter, maybe 1m high. The balls were about 25mm diameter. Apparently for this to work you need sand with very sharp-angled grains.

Last edited by a moderator: Sep 25, 2014
6. Jun 27, 2012

### DennisN

Hi! A cloud chamber would certainly be cool. Otherwise there's of course also the famous double-slit experiment, it's quite easy to do today. As a matter of fact, I experimented with this some months ago at home for fun, trying to get the best result with minimal equipment. I was thinking of writing a short guide on this actually, but I haven't done it yet. But here's what you would need:

1. A laser
2. Slits
3. A screen

Some notes on the way:

I used a small pen laser (at first with the power button pressed down by tape or something, but later I disassembled the laser and used an external power supply and a power switch).

There are a number of ways to construct slits; I have tried these (and I preferred option 2):

1. Razorblades and aluminium foil. Paint the aluminium foil black with e.g. spray paint. Glue/tape two razorblades together and cut two parallel slits in the aluminium foil (actually make a number of slit setups, since it's quite easy to tear apart the foil). This is however not the solution I preferred, it's a little tricky and easy to mess up.

2. One "barrier" is equivalent to two slits with infinite width. So you can get an interference pattern by using just a single thin barrier. You can use a hair (40-120 μm) taped up on some frame (I used old dia slides). You can also use a thin copper wire extracted from an electrical wire (paint it black). You can actually also use a lead for mechanical pencils; I tried with a 0.5 mm lead and managed to get a pattern nevertheless. But thinner barriers are better; I preferred hair or copper wire.

3. You can also take one or a couple of needles, paint them black and tape them together. But I prefer option 2.

4. Reflection from a laser via a CD is also supposed to show an interference pattern. I've tried this, but got pretty strange patterns which I found difficult to interpret.

5. You could also buy a slit setup from some science shop.

There are some other issues to address as well; laser targeting, environment lighting and screen distance. It's a little tricky to point the laser perfectly at the slits, so some smart mechanical setup is recommended. The laser can be fixated with e.g. tape or something, but I actually used a small adjustable Lego Technic construction I built for this purpose :). Further, in a bright room the interference pattern can be hard to distinguish. One solution is to "screen off" the screen, e.g. tape up a screen inside a cardboard box or something (the screen can be a simple white paper). And last, the distance from slits to screen will determine the pattern resolution (longer distance->wider pattern). I got the best result with the screen at about 4 meters from the slits. If the screen is too far away, I found the pattern to get too blurry.

I attach some pics from my experiment (I apologize for a lousy camera used by a mediocre photographer);

1. My first crappy setup (but it worked).
2. An adjustable Lego Technic laser cannon.
3. Interference pattern.
4. Interference pattern from shining laser through a thin black cloth. Irregular vertical/horizontal pattern since the threads in the cloth are not perfectly parallel.
5. Pattern (?) from a CD (I can't really interpret that one ).

Here's an instructional video I found (apparently there are special CDs with slits too);

There's also the quantum eraser, but this is somewhat more tricky than the doubleslit setup;
Do-It-Yourself Quantum Eraser:
http://www.scientificamerican.com/slideshow.cfm?id=a-do-it-yourself-quantum-eraser

I haven't tried that, but I probably will sometime.

#### Attached Files:

File size:
24 KB
Views:
194
File size:
35.1 KB
Views:
183
• ###### 3-Interference-Closer.jpg
File size:
2.6 KB
Views:
188
Last edited: Jun 27, 2012
7. Jun 27, 2012

### DennisN

The two other pics of mine;

#### Attached Files:

File size:
2.4 KB
Views:
185
• ###### 5-CD4points.jpg
File size:
37.9 KB
Views:
189
8. Jun 27, 2012

### M Quack

9. Jun 27, 2012

### QuantumPion

A cloud chamber will detect any ionizing radiation, including muons and positrons. If you put a magnetic field in the cloud chamber than you can differentiate between positrons and electrons.

10. Jun 28, 2012

### elishamei9000

AWESOME SUGGESTION! how much did it cost you? and how much time did it consumed for you to make one?

11. Jun 28, 2012

### DennisN

Hi, I'm glad you liked it! The first crappy setup cost me nothing, as I already had the necessary stuff at home. And that setup took me an hour or so (incl. doing slits). I have done the experiment many years before with professional equipment at university, so I already had some experience.

Pen lasers are cheap; 10$or so (the laser I used is a cheap, crappy one). If you have nothing at all to start with, I think you can make it with less than 20$ (laser, slits, tape/Blu-Tack, tools).

A friend of mine got the idea that Lego Technic could be useful for this purpose. I bought a Lego Technic Crawler Crane (9391) for about 20\$ and tried to build an adjustable laser device out of it. But this took me some time though, since I had to figure out a workable design. It took me probably four hours or so (experimenting, building).

In order to connect an external power supply to a pen laser, you'll need some tools (soldering) and basic electronics knowledge. I used a 4,5 V/300 mA adapter + one capacitor (220 μF, 50V) in parallel (for extra voltage stabilization) and a 330 Ω resistor in series (to get a proper current) + a power switch. This took me about an hour or so. But, depending on the laser and the adapter, this can take some time and experimenting in order to get a good voltage/current (not too high, not too low); the brightness and quality of the laser light is very dependent on this.

It's really all up to you, how much time you want to spend and what you think you'll be able to manage. As I said, you can do a very basic experiment with a pen laser, tape/Blu-Tack and pencil leads/hair/thin copper wire, it's quite easy.

I found two more videos;

Double Slit Experiment with just a laser and three pieces of pencil lead

Double Slit Experiment (How to do it at home)
(he got better patterns than me; it seems his laser was better than mine and that he used a nice, really dark environment)

But I think a cloud chamber would be quite cool too; I got inspired and will probably try that out sometime . Anyway, I wish you good luck!

Last edited by a moderator: May 6, 2017
12. Jun 29, 2012

### Karimspencer

I made a nice blog post on how to make a laser, it's simple and fun.

Check it out : https://www.physicsforums.com/blog.php?b=4067 [Broken]

Last edited by a moderator: May 6, 2017