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DEvens

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## Summary:

- Wave function of a laser beam before it hits the diffraction grating

## Main Question or Discussion Point

**Summary::**Wave function of a laser beam before it hits the diffraction grating

So I'm reading "Foundations of Quantum Mechanics" by Travis Norsen. And I've just read Section 2.4 on diffraction and interference. And he derives a lovely formula for the wave function of a particle after it leaves a Gaussian diffraction slit. Then he puts two slits side-by-side and gets the well-known double slit formula. There's even a lovely color picture of the wave function after the double-slit and it gives a very clear picture of how the interference pattern develops. All very pretty.

I'm wondering about the wave function before the double slit. What kind of wave function is it that allows a laser beam to move along without spreading out, or not very much. I fondly remember from high school, we had a Helium-Neon laser that, with no special optics, could send a beam 100 meters with no noticeable spread. Yet a few cm after the diffraction grating it had spread many cm wide.

Norsen has plane waves before the slits. So that's kind of not what I'm looking for. My laser in high school had a very narrow beam, maybe half a cm across.

What's the wave function of a non-spreading beam? Or even a nearly non-spreading beam? How can I get a narrow circular beam that does not spread out?