I was reading about quantum physics recently, and came across what is knows as the ‘two slit experiment’.
I performed my own version of this experiment using a piece of paper with one, then two slits, and a torch. It resulted in two beams of light in the shape of the slits hitting the end surface. I tried again using varying widths for the slits, varying distance of the light source… nowhere did I see any interference pattern, just two strips of light in the shape of the slits.
What’s going on?
Perhaps you need something better than a torch and some paper to perform the experiment accurately?
Hmm… I think if the slits are too wide it won’t work. I think they have to be pretty narrow. Try to make the slits narrower and not too far apart. You may have to experiment.
Your problem is probably the width of the slits but, out of curiosity:
[ul]
[li]What are you using for the end surface?[/li][li]Are you using any kind of measuring or sensing device that might, even inadvertently, observe which slit a photon goes through?[/li][/ul]
Two Slit Wikipedia article explains the conditions for interference. Your torch is probably not acting as a point source, so try reducing it to a pinhole.
Si
The spacing between the slits has to be very narrow indeed - less than a millimetre. I doubt if you could do it easily with two slits cut in a piece of paper. When I was at school the demonstration used two scratches on a piece of smoked glass.
Also you need a monochromatic light source - daylight or the light from a tungsten filament bulb won’t do as it consists of a whole range of frequencies.
A laser is a good choice. 
You DON’T need a monochromatic source. If your slits are close enough together you’ll get excellent multicolor fringes. But a laser is much easier to use to get good results, not only because it’s monochromatic, but also because it’s got a very high directed intensity and (arguably most important of all) it has a very large coherence length. You’ll get a lot more orders of interference (light and dark bands) with a laser than with any other light source.
What we used in a physics lab many years ago was a smoked microscope slide. Two razor blades pressed together were used to make slits in the smoke. Our light source was a regular bulb with a piece of red cellophane in front of it.
The proper experiment uses a laser. Your torch (“flashlight” to us on this side of the pond?) is emitting a whole lot of photons, but they aren’t really correlated with each other at all. A laser emits a lot of photons that are all exactly the same, and that’s why they interfere with each other the right way.
The original experiment in 1805 used sunlight - they didn’t have lasers (alright, they had prisms and maybe diffraction gratings). It may be easier and look better with a laser, but it is not required. But the source does need to be a pinhole, a wide light source creates multiple diffractions and they all smear over one another.
Two Slit Wikipedia article explains all quite clearly.
Si
The light in a torch/flashlight or the sun is correlated to a small degree – that’s the “coherence length” I mentioned in my post above. In fact, you can use thjis experiment to measure the coherence length of a light source. It’s just that the coherence length of a laseer is so much larger that it’s easier to do the experiment with a laser.
As si notes, Thomas Young did this circa 1805, using sunlight. It’s best to use sunlight that’s gone through a small hole first, to restrict its angular extent.
By the way, although Young explains the experiment, and gives a diagram, his experiment differs somewhat from the “classic” two-slit experiment as performed in school demos. In fact, he never actually says he did the experiment at all, so he may or may not have performed it.
Mea culpa. IANA Experimental Physicist, and the descriptions I recalled are all in terms of lasers. Yes, any light source should work, but I’m not surprised that the fringes are so small in the OP’s setup as to be unnoticeable.