# Optimal slit width in Young's double slit experiment

I'm trying to do Young's double slit experiment at home.

Note that I don't have a laser, only a torch. I could get a bulb or use a candle though, if it helps

I built the slits by cutting into a black chart paper with a knife.

I tried to build a setup by placing a single slit, double slits and a screen one after the other, and shine a torch through the single slit. I tried varying distance between single-slit/double-slit and double-slit/screen which did not help. I did not observe any interference pattern. All I got was two parallel bright fringes instead, like the ones you would get by shining a torch through two very thick slits.

I think that is precisely the problem, that the slits are pretty thick. What is the optimal slit width I should have to observe an interference pattern? How do I build that with a chart paper?

I have seen videos of this experiment online, where people use pencil leads and hair(!!), but they both use a laser. I don't want to use a laser(just because I don't have one, and I'm probably too lazy to go and get one!).

Note that you may suggest using any other materials that might be easily available at home.

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it doesn't have to be a mask with two slits cut in. i have replicated this experiment by shining a laser pointer through the gaps of 3 needles that i held between my thumb and forefinger. –  John Cuna Sep 6 '13 at 16:21
Just a tip on performing this experiment in general: You should calculate beforehand the size of the pattern you expect to see. This will (1) convince you that you really need monochromatic light, laser or not, and (2) give you a sense for what lengths to use and what to look for. For example, there are two patterns you might find on the screen, one based on $\lambda$/(slit separation), the other based on $\lambda$/(slit width), and if you don't know the approximate size of what you expect to see, it can be very hard to get a visible pattern. –  Chris White Sep 6 '13 at 21:32
Have you seen how the person in this video sets up the double slit experiment? Skip to 2:00. He uses a large cardboard box and is able to get good results on a sunny day. –  Noah Sep 8 '13 at 0:59
I have seen it. Coincidentally, that video is what inspired me to do the experiment myself. Unfortunately he uses manufactured slits(I can see they are pretty close together and thin). –  udiboy1209 Sep 8 '13 at 7:34
I've heard that one can use a candle to deposit a layer of black soot on a glass slide and then use a pair of single edge razors stacked together to make the double slit. I tried this once and it was rather smudge-y. It's easy to see interference if you hold slits or pinched fingers up to your eye and look at a long filament bulb. It's harder to see that pattern projected on a surface. –  Noah Sep 8 '13 at 18:27

All I got was two parallel bright fringes instead, like the ones you would get by shining a torch through two very thick slits.

It means the separation between slit is not close and the slits size is not small enough! Those two light beam must overlap to have interference. Small slit size is required to have large diffraction, the optimal slit size is certainly small than wavelength $\lesssim\lambda\approx0.5\mu m$ which gives you large diffraction. However, larger slits size is ok, but you have to (a) Make two slits as close as possible (b) move the setup far from the screen. You will know that it is enough when the light beam can overlap.

For the slit, you need better tools than a knife as well as a better material. First, you should use a shape cutter. Second, you need a material that can have a sharper edge such as film. I believe that film was used in the first few experiments of this kind. You have mentioned a hair is enough so $10\mu m$ should probably be ok, you just need to move the screen further away.

For the light source, you should always use a laser, since a high coherent light is required. Any laser out there is ok, it just cost 1 dollar and I can sure you can borrow a laser pointer near you. As I remember when I was doing Michelson Morley experiment, a tungsten light only gives interference pattern for $<0.1m$ with short coherent length, but a laser can have coherent length $>2m$. It means your life can be easier as you can use a 20 times larger slit with a laser!

Edit: Additional info on the methods Young used for this experiment.

The wiki about Young' interference experiment has quoted his paper on "On the nature of light and colours" (Also around page p.140 in the book Method and Appraisal in the Physical Sciences). The relevant excerpt is:

In order that the effects of two portions of light may be thus combined, it is necessary that they be derived from the same origin, and that they arrive at the same point by different paths, in directions not much deviating from each other. This deviation may be produced in one or both of the portions by diffraction, by reflection, by refraction, or by any of these effects combined; but the simplest case appears to be, when a beam of homogeneous light falls on a screen in which there are two very small holes or slits, which may be considered as centres of divergence, from whence the light is diffracted in every direction.

So, I guess the experiments were carried out as follow:

1. Light source: In a room with all windows covered with thick curtains so that the inside is completely dark. Then let a small beam of sunlight go in.
2. Monochromatic light: Use prism to split light into different color (This is known method back to Netwon). To get high quality single frequency light, a slit is required in front of prism to get a narrow sunlight beam.
3. Point source of monochromatic light: Add another slit to get the required color (S1 in Fig. 1), the output monochromatic light is therefore from a single point source.
4. Interference: Add another double slits (S2 in Fig. 1) so that the light can have two different path. Make sure that light from S1 falls on the slits S2. To ease observations, the screen should be far away.

Since his results cover all color, so it is very likely that he used sunlight rather than other light source such as candle (There was no light bulk at that time). Also, there is no diffraction grating, so it is likely that he was just using a simple prism.

For home experiments carried out these day, we can use LED as a monochromatic light source so that step 1 and 2 can be skipped. If you use a torch, you still need the step 2.

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"only give interference for <0.1m and a laser give >2m", what quantity do you refer to with "0.1m" and "2m"? distance between source and slit? –  udiboy1209 Aug 18 '13 at 5:21
@udiboy I refer the distance that you can observe the interference patterns, i.e. coherent length here. Updated –  hwlau Aug 18 '13 at 5:43
Seems as though I'll have to get a laser anyway. :/ –  udiboy1209 Aug 18 '13 at 5:48
@udiboy Yes, is it really that hard? –  hwlau Aug 18 '13 at 5:48
@udiboy I am not quite sure how he did the experiment. But at least he should have filtered out white light except a single frequency, which is similar to an LED. And he should have special method to prepare the firm. I think... –  hwlau Aug 18 '13 at 5:56

I am using a super bright LED torch, and wow! It even works with pin holes instead of slits, the interference pattern is round and uniform. It also works with one vertical slit and one horizontal slit. With the LED there also seems to be a 'solid' looking 'bar' at the centre of the pattern.

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How far apart were the pinholes/slits ? and what equipment did you use to make them, what kind of film etc.? Also could you post pictures, although that's not really necessary –  udiboy1209 Apr 29 at 11:20