Absolutely Small, Chapter 6: How Big Is a Photon and the Heisenberg Uncertainty Principle by Michael D. FAYER
This chapter lays out how things that we call particles actually are both particles and waves. Whether you think something is a particle or a wave depends on how you look at it, that is, what experiment you do. These ideas are formalized in terms of the Heisenberg Uncertainty Principle. The Uncertainty Principle is fundamental to quantum theory. It tells us what we can and cannot know about an absolutely small particle such as an electron or atom. The true description of nature, embodied in the Uncertainty Principle, is wildly different from classical mechanics.