Explain why a laser beam can't be perfectly parallel, with no spreading.
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Step 1: A laser beam is made up of light waves. Show more…
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A beam of light always spreads out. Why can a beam not be created with parallel rays to prevent spreading? Why can lenses, mirrors, or apertures not be used to correct the spreading?
Ankur S.
Show that the diffractive spread of a laser beam, $\approx \lambda / D$ as described in Section 9 of "Quantum Mechanics of Atoms", is precisely what you might expect from the uncertainty principle. [Hint: since the beam's width is constrained by the dimension of the aperture $D,$ the component of the light's momentum perpendicular to the laser axis is uncertain.
If Huygens's Principle holds everywhere, why does a laser beam not spread out? a) All the light waves that spread in the perpendicular direction from the beam interfere destructively. b) It does spread out, but the spread is so small that we don't notice it. c) Huygens's Principle isn't true in general; it only applies to slits, edges, and other obstacles. d) Lasers employ additional special beams to keep the main beam from spreading.
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