To the first question, an “observation” in this context is something that determines whether a photon has gone through a particular slit or multiple slits. Such an observation changes the behavior of the photon by having it interact with some kind of detector or “observerer.” People keep saying that you would use a camera of some sort. These people haven’t thought it through. A camera has a low probability of detecting a photon for a lot of detailed reasons I won’t go into here, but cameras which detect the photon take the photon out of play. (It is absorbed.) The trick is to observe the photon without absorbing it. That takes some doing. But it can be done. And once it is done, it doesn’t matter whether any human know about it or takes it into account. The photon has been disturbed. It has been “observed” by a small particle of some kind.

The second question has to do with determining whether light is a particle or a wave. It isn’t so there is no determination to be made. We can define a new thing sort of like a particle, but not really, and call it something like a QP and say it is that. But we can’t say it is a particle or a wave with the meaning of the words as we know them.

The issue here is that nature behaves the way she does and she doesn’t care whether we understand it or not, nor what we think. Light is neither particle nor wave in any way that you think of macroscopic particles or waves.

You tend to think of waves having energy that you could sub-divide as many times as you wish and find the energy spread out over a large area.

By observing light carefully through many experiments, we find that it just doesn’t fit into either category fully. We go looking for small amount of light and it doesn’t seem to be spread out over the wave. It seems to pop up in little packets that appear large for the area affected. It is almost as if the energy in the wave concentrates down into a single electron.

Further, it doesn’t seem like a particle either, since it can go through two slits and interfere with itself.

So, it is either some new kind of particle that has a wave function to it, or it is some kind of a wave that can collapse down into an energy packet absorbed at a single electron. Our classical way of thinking about particles and waves doesn’t fit this.

So for a long time, we talked about the particle-wave duality. Then we embraced QED (quantum electro-dynamics) which said that light is made of particles, but that the particles have a probability wave function, and the particles somehow don’t behave like particles. They don’t take one path or the other and we can’t say they take both paths, or we refuse to. Yet mathematically, we have to say that the photon takes all paths with different probabilities and we integrate all the probabilities (with some hocus pocus to get around the fact that the integrals contain infinities.)

So these really don’t resemble any particles that you have any experience with.

And then we started to say, maybe they are none of the above. Maybe they are just excitations in the quantum field.

Let me repeat. Nature doesn’t care what you call it or think about it. It is just doing its thing. And photons do not act like any wave or particle that you know of. If you call them a particles or a waves, it is probably going to get in the way of understanding light.

It seems like you are asking, if we could take a smart pill, if we could say whether it is a wave or a particle. No, we know it is neither.

For some work that I do in lasers, I pretend light is a wave, because I know that the math will give me the right answer. But in other problems, I pretend light is particles, because I know the math will work. And in very low light, I treat the detection of the light as Poisson statistics.

If you insist that light be either a particle or a wave, you will never get an answer and you will never understand light.