# Question about heisenberg uncertainty principle and single-slit diffraction

My question relates to this video illustrating the Heisenberg uncertainty principle.

The video shows light being shot through a slit forming a spot on a screen. As the slit narrows, the width of the spot narrows, until eventually the width of the spot starts increasing. The person in the video says this is counterintuitive.

• I'm slightly puzzled as to why this is counterintuitive. Isn't this just like classical single-slit Fraunhofer diffraction, which would work with water waves?

• So what I'm guessing is the experiment is counter-intuitive using a particle view of light?

• So does this mean Heisenberg's Uncertainty principle is somehow equivalent to wave-particle duality?

Looking to clarify if my thinking is correct.

• Basically, yes. The uncertainty principle applies only to waves. The results are supposed to be unintuitive because light is often thought of as a particle. – knzhou Jan 17 '16 at 18:57
• However, people often talk about light as a wave, too, so it might have been more intuitive for you. But consider that the double slit experiment also works on electrons! – knzhou Jan 17 '16 at 18:58
• When you post questions here, always include all information you need. Linking a video is ok if it's supporting information, but if it's central to the question please include all necessary information directly in the post. – DanielSank Jan 17 '16 at 19:23
• It's not counterintuitive until someone tells you to think of photons and electrons as particles (which they are not). If you don't follow them down that path of poor thinking, the uncertainty principle is a trivial consequence of (linear) field theories. If you stay with field theories, then you have to explain how it is possible that we can see "particles" in the world, but that has successfully been explained as soon as 1929, most people just don't know the correct explanation. – CuriousOne Jan 17 '16 at 19:54
• @CuriousOne And you can't possibly get into (quantum) field theories unless you get a good grip on the uncertainty principle and all it entails in the basics of QM o_o. The diffraction picture is counterintuitive only if there is a lack of background in classical wave physics. Otherwise it is as the OP rightly observes, a simple diffraction experiment that provides an excellent intuitive and didactic illustration for the Uncertainty Principle. – udrv Jan 17 '16 at 20:29