What makes the light waves in laser light parallel?
Category: Physics Published: December 20, 2012
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
The waves in laser light are not parallel. It is theoretically impossible to construct a beam with perfectly parallel rays unless you have an infinitely wide beam. As described in the textbook "Principles of Lasers" by Orazio Svelto, even a perfectly spatially coherent beam will spread out due to diffraction. Diffraction means that all waves – including sound, water, radio, and light – bend around corners. And it's not just the edge of the wave that bends around the corner. It is the entire wave. This means that a beam of light that is shone through a hole spreads out as it travels. A beam with perfectly parallel rays would never spread out. Every beam of light has a finite beam width and therefore can be thought of as emanating from a hole. Diffraction is a wave effect, so it applies to laser beams as well.
The divergence of a beam means the amount that the rays are spreading out. The amount that they spread out depends on the length of the waves, and the width of the beam. A narrower beam of laser light spreads out more quickly than a wider beam. Only an infinitely wide beam (a plane wave) does not spread out, and therefore has parallel waves. Divergence is observed by shining a laser light on a wall and then seeing the spot grow bigger as you move away from the wall. Also, it is inaccurate to speak about a laser beam having multiple rays or multiple waves. A laser beam is one wave, with different parts of the wave going in different directions, much like the water ripples created by throwing a rock in a pond. That is the interesting thing about photons, which are the smallest building blocks of light: they can join the same wave state. The fundamental difference between laser light and light from a regular bulb is not that the one is parallel and the other is random. Rather, laser light is one wave (many photons in the same wave state), while regular light is many waves mixed together somewhat randomly. It is true though that the laser light tends be more parallel than regular light if prepared properly.