If you want proof, there is a thing called photomultiplier. A photomultiplier counts photons. You can’t count waves, but you can count particles.
What concerns us about light is how it behaves, because physicists always like to know why things happen, and to figure this out, they need to know how things happen. Some daily-life examples from light’s behavior is a good place to start observing light. We know that light gets reflected from a mirror with the same angle that it hits the mirror, light travels in a straight line, light bends when it passes through the water, light can be separated into different colors, and we can focus light with a lens.
From our observations, we know a lot about how light behaves. Now, we can think about why light behaves like this. Do you know who knows the answer? Well, not Richard Feynman. Unfortunately, I cannot tell you why light behaves the way it does because just like Feynman, I don’t know the answer as well.
“I am not going to explain how the photons actually “decide” whether to bounce back or go through; that is not known. (Probably the question has no meaning.) I will only show you how to calculate the correct probability that light will be reflected from glass of a given thickness, because that’s the only thing physicists know how to do!” (Feynman 24).
The concept of Causality might be helpful to understand what Feynman tries to say. Causality is the relationship between cause and effect. All dominos fall because you hit the first one. Can you imagine a single domino falling on its own, without a cause? No wind, no force, no friction or anything. I believe you can’t. Because you know that everything happens because of a cause. Before learning about quantum mechanics I was 99.9% sure that everything happening in our universe had a cause, but now, I am only 99 % sure, because there is something that makes me question causality, and here is it. Image from QED: The Strange Theory of Light and Matter
Consider an experiment in which you put a photomultiplier in front of a light source and another to the place that you expect the light to reflect through a piece of glass. A piece of glass passes through most of the light that hits it but some of the light is reflected. In fact, 96 % passed through and 4 % was reflected in this experiment. You probably don’t see any problem with this, but there is a problem. Imagine that you are a single photon. You came out of the light source and you hit the surface of the glass. What will you do? Will you go to the photomultiplier A or B? Statistically, you go to A 4% of the time and B 96% of the time, but why? What happens there? As I told you, we don’t know but we can at least know something else: we can know how much of the light would be reflected, 4%. And as far as the experiments show us, we can also know how much of the light will be reflected if we thicken the glass.