C30: If the Huygens-Fresnel principle applies to any wavefront anywhere in a beam path, why doesn't a laser beam without any slit spread out in all directions?

1. Because all waves that spread interfere destructively.

2. It does spread, but the spread is so small that we normally don't notice it.

3. We can't apply the Huygens-Fresnel principle anywhere but in slits and apertures.