Why are there two slits in the double-slit experiment?
When the light passes through a (very) small gap, it diffracts, or spreads out in an arc. With two slits, these arcs overlap, which causes the individual waves to interfere with each other, some constructively, and others destructively. When this hits a screen, the interference causes light and dark bands on the screen. The light bands are formed when the waves interfere constructively, and the dark bands where they interfere destructively. You can see how this happens by using a ripple tank with two oscillators in, which should cause the interference pattern that would occur with the light. If you used one slit, the light would diffract, but this would just be like shining a normal light on the screen, you would not get the light and dark bars. However, you could use more than two slits, but this would cause a huge inteference pattern that would cause the light and dark bars to be bunched very closly together on the screen, and it would be much more difficult to calulate the frequency
