This https://www.gentec-eo.com/blog/spot-size-of-laser-beam article states that laser beam width changes along the length of the ray (the function stated doesn’t look like it could be a constant 1)

Is it impossible to create optics that completely parallelize laser light? If not, why so?

  • $\begingroup$ Every laser beam will suffer the effects of diffraction - therefore, it is impossible to have a beam with constant width. You can collimate it to decrease this effect, but it will always be present. $\endgroup$
    – Woe
    Jun 1, 2019 at 22:40
  • $\begingroup$ Is there some formula then that gives the minimal angle possible? $\endgroup$ Jun 1, 2019 at 23:11

1 Answer 1


You can’t have a perfectly parallel beam. There are two equivalent ways to see this.

The uncertainty principle: the photons making up the beam have to have some spread (uncertainty) in their transverse momentum because they’re confined in position to a certain beam width.

Diffraction: by creating a beam of a certain width, you must have a certain amount of diffraction that causes the beam to diverge.

Fundamentally, these are the same. They come from the wave nature of light.

  • $\begingroup$ Simplified, a laser pointer creates a sharp light circle with some stray glow around it. Does diffraction mean the circle part is bound to grow, or is it possible to keep the circle constant with and it just gets more faint with distance and the glow around it gets more prominent? $\endgroup$ Jun 2, 2019 at 7:42
  • $\begingroup$ The central part will grow, because the beam is diverging. $\endgroup$ Jun 2, 2019 at 15:55
  • $\begingroup$ But if I can focus a beam into a coverging beam using a les, why can’t I focus a beam to infinity so that divergance is zero? $\endgroup$ Jun 6, 2019 at 7:44
  • $\begingroup$ Because there is no such thing as perfectly parallel light. See answer for physical reasons why. $\endgroup$ Jun 6, 2019 at 15:10
  • $\begingroup$ Experimental proof: get a long focal length lens, go outaide, and try to make a point image from sunlight. You can’t. The light from the left side of the sun and the right side of the sun follow non-parallel, diverging paths. The longer your lens, the bigger the spot (image of the sun) will be. $\endgroup$ Jun 6, 2019 at 15:40

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