At night, I went outside. I had a box with two slits in it. I directed torch light towards it, but I saw only two bands of light on the wall and shadow from the rest of the box. Why did it not produce interference like a double slit experiment should?
Answer
In order to see the interference fringes, four conditions must be fulfilled:
Your light source either has to be point-like or very far away from the slits,
Your light source must be monochromatic* (i.e. emit only at a single wavelength),
The slits must be very close together, and
The slits must be very thin.
Failing to meet any of these will generate enough noise to completely obscure any signal you hope to measure. Probably the first one to fix in your case is to switch from a torch (which is neither point-like nor monochromatic) to a laser (which is much closer to being both of those things).
*In order to see the usual interference fringes, you must use monochromatic light. There is a similar effect you can do with a point source of white light, though, as shown in the comments.
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