Heisenberg's uncertainty principle?

The Uncertainty Principle is not just a property of quantum particles, but of waves in general (and of particles that behave like waves, such as photons and electrons). Imagine you have a wave moving through empty space. Measuring its speed can be done with a high degree of precision, but where IS the wave? Well, it's sort of stretched out over a region of space and hard to localize. It would be much easier to locate the wave if it were a single pulse, but then you run into another problem; measuring the speed of that wave is impossible because you don't have repeated pulses to measure.

That is an incredibly simplified view of the HUP, but it gets you in the door. The more precisely you know a wave's position, the less precisely you can know its speed. If the wave is a wavelike particle - like an electron or a photon - then that uncertainty in speed translates into an uncertainty in momentum.

You may remember that momentum is a vector quantity; it has both a magnitude and a direction. This explains why the laser beam spread out as the gap narrowed. By narrowing the gap, you've increased the certainty of the photon's positions; in other words, you can tell with increasing certainty where each photon must have passed as it went through the gap. The trade-off is that each photon's momentum (which includes its direction) becomes LESS certain; ergo, the area over which the photons strike the screen widens.

Now keep in mind that this is a pretty simplified explanation; I'm really just trying to give you a mental model of what's going on as the laser passes through the gap. Still, I hope it helps. Good luck!

EDIT: Steve is describing the Observer Effect, not the Heisenberg Uncertainty Principle. They are often confused, but they are not the same thing.

You can't measure anything without changing it.

the more you know about one thing the less you know about the other.