There appears to be a strong and continuing miconception about light/photons as far as this so-called "duality" is concerned. Let's get ONE thing straight here - Quantum mechanics does NOT have two separate descriptions of light for when it behaves as a "wave" and when it behaves as "particles". PERIOD! It has one, and only one, consistent description for light, and that's that.
Now, after reading that, would one still want to consider light as having a "wave-particle duality"?
From the way I see it, the continuing misconception here is due to the ambiguity of the quality used in the question. We apply our classical ideas of what "wave" is, and what a "particle" is. A particle, like a grain of sand, has a definite boundary in space, i.e. a grain of sand doesn't appear spread out that it's exact shape and boundary are vague. Thus, it has what we classically define as a particle. A wave, on the other hand, can spread out over space.
Now, a photon, as a particle, was NEVER defined this way! A photon description in QM is NOT defined as having an exact shape and boundary in space. It is defined as clumps of energy. So in energy coordinates, it has definite "points", but it has no definite "size" in real space! This isn't your classical particle.
Having said that, the most common explanation for the "wave-particle duality" is that light behaves as waves in experiments such as the double slit, and behaves as particles when we do things like the photoelectric effect. Now, the fact that it is EASIER to describe an observation using one type of description while describing another observation using another type of description does NOT mean that they can't be described using ONE consistent discription. Most people often do not realize that one CAN describe interference effects (a typical wave phenomena) using photons![1] In fact, such technique CLEARLY explain diffraction patterns, and how the uncertainty principle is clearly at work. We don't normally subject students to such things because it is MORE involved than using the simple wave description. But we should not fool ourselves into thinking that the photon picture cannot be used to arrive at the idential phenomena that once thought can ONLY be described using the wave picture.
Again, one needs to learn QM and realize that there are no separate description for this wave-particle duality illusion. It is only a duality based on our pre-existing prejudice that something must either be a wave, or a particle. This "duality" thing only appears to be a major "issue" in pop-sci books and articles. It is a non-issue in QM texts.
Zz.
[1] T. Marcella, Eur. J. Phys., v.23, p.615 (2002).
