What are the conditions under which light will behave like waves, and ditto for particles?
Well, detailed description of wave-particle duality requires more physics tools that we have for this course, but I can try to give some feeling for it. There is actually no precise point at which light (or matter) stops being a particle and turns into a wave: there is kind of a continuous range of being “more particle-like” to being “more wave-like”. There is a principle of quantum mechanics called the “Heisenberg uncertainty principle”, which says that the more uncertain something’s position is, the less uncertain its momentum is, and vice versa. Something with a precise position is “localized”, and behaves like a particle; it then has a poorly defined momentum. Something with a precise momentum but poorly defined position behaves like a wave (a wave carries a specific momentum, but is spread out in space). Light (or matter) can exist with more or less localization, in a continuous range. We can “force” light to be localized by making it go through a small slit (for example
