13.2 Wave-Particle Duality

Alright, in today's lesson, we're going to look at two different models of EMR (Electromagnetic Radiation) and how it travels. In any topic, scientists always try to come up with new ideas, prove them, refine existing ideas, and try to make things better. They also try to disprove different theories.EMR, especially light, is a topic that has undergone lots of battles between proving and disproving. It has divided the scientific community. So, what we're going to do is go back in time to see how scientists thought light worked and how it has changed to what it is today. There are basically two different groups we're going to look at. The first is the particle model, which believes that light is like a little particle, similar to a ball that you would throw, traveling to pass on energy. The second model we're going to look at is Light travels as a wave, so just like we looked at waves in Physics 20, we're gonna think of it that way. So, is it a particle or is it a wave? That's what we're looking at today. We're gonna start with our master of physics, good old buddy Isaac Newton. He was the one who first proposed the particle model, and because it was Newton, because he was right about a lot of things, the science world backed him up and agreed. Newton had some pretty solid theories and proof of light as a particle. He said light travels in straight lines, just like when you throw a ball and it travels in a straight line unless there's another force acting on it, like gravity. And if we shine a flashlight on somebody, it creates a shadow, so obviously the light is going in a straight line, and the person or object blocks out those beams of light. So, therefore, think of those little particles. They would hit an obstacle like a person, and that doesn't pass through. That's why we get the shadows. And he also said particles can be reflected. So if you throw a ball and it bounces off the wall, it comes back at you. Like that's the same thing with a mirror. It bounces back. And it's probably the biggest proof of why it was a particle. We knew that waves had to have a medium. Waves had to travel through something. It has to travel through water to get water waves. It has to travel in a rope to make a wave on a skipping rope, and so on. And he said light coming from the Sun has to be then a particle because how would it have traveled through space if there was no medium? Space is just basically a vacuum. Therefore, it has to be a particle that could just travel through that empty void. Later on, some scientists started to say that while Maybe it was about quite right, and the first was Christian Huygens. What he looked at was sort of Newton's idea that light has to travel in a straight line. That's why we get shadows and things like that. He started looking at a shadow around an object. He saw that the edges were sort of blurry. So he was wondering why, why would they be blurry if the light was traveling in a straight line?It would be kind of like imagine if you make a stencil and you spray paint around it. So, if I had a cutout of the letter "A" and I spray-painted around it, all the paint would go outside the "A." And then, where the letter "A" would be, that would block the paint. If we lifted it off the paper, we'd have a perfect "A" mark from where the paint didn't touch. So he was saying, if it was particles, that's what it would look like. But if we actually look at a shadow, the edges are kind of blurry and fuzzy. So what he did. Was he started looking at better quality shadows. So what they did was they would use a high-intensity light and shine it on a disk to see what kind of shadows were created and to try to see if there are any patterns. Their logic is that light is actually bending around the shadow, and we're getting sort of interference when it goes around. Huygens actually proved this to be true. So, this little picture I have at the very bottom is a shadow pattern that's created with a high-intensity light around a solid disc. Because the waves are interfering and we have constructive and destructive interference, we get these kind of weird little patterns going on.Earlier, there was a French scientist named Simon Poisson and he predicted. That light had to be a particle because there's no way we'd get shadows around the disc," and he said, "Well, if they actually did, you'd end up getting a dot in the middle where there would be constructive interference, and I've never seen that happen, so it has to be a particle." Huygens and guys who believed in the wave model actually proved that Poisson was totally wrong. A guy named Dominic Arreaga later on disproved what Poisson was saying. He actually did it with a pretty intense, high-powered light, and we got the picture that we see at the bottom. So they kind of named it the Poisson spot to say, "Well, because he was such an idiot and he was totally wrong," they kind of use that as a joke. There are still some people saying Huygens wasn't right, and part of it we were thinking, "Well, how does that work when water, or say, when light hits water?" We get glare reflection off the water, but some of the light actually passes through. So, what Huygens actually did, he said, "Well, it has to be a wave." He measured this in an experiment. So, he said, "If we had an incident wave, let's suppose it was a wave with an amplitude of five." He then measured how much of the wave got reflected. Let's suppose two got reflected, and then he measured what went through, which was transmitted into water, and it was three. What he found out was that the original five broke up into two equal parts that added up to the original. He said it acted just like when we have a wave in a fence. So, if you remember in physics 20, we had a wave of three and another wave of two approaching each other. They would join for a wave of five, and then back up into the three and two. He was saying it did the exact same thing when he did this with light. Therefore, it had to be a wave. They then approached Newton to sort of explain how did that work, and he came up with a pretty lame explanation. Explanation: He said that the particles that come in collide with the water, causing them to become excited and bounce around. Some of the particles decide to be reflected, while others pass through. This response was so weak that Newton lost support for his particle model. Many years later, people began to follow the wave model, inspired by Huygens' ideas. Then, a man named Thomas Young conducted a double slit experiment, which we will discuss later. In this experiment, small microscopic slits were cut using razor blades, and light was shone through them, resulting in interference patterns similar to those observed in water. Waves on the other side, so because of that, we got these highs and lows and bright spots, and so on. Thomas Young said, "Well, it has to be a wave if we're getting wave interference." And from that on, pretty much for about a hundred years, everybody accepted the wave model. What happened though, later on (and this is like late 1800s, early 1900s), now other scientists came on and said, "Well, a lot of seconds maybe might have been onto something." And the first was Max Planck. We're gonna talk about him later, but he sort of came up with what he called blackbody radiation, and he said that light and the energy, the amount that was being absorbed, went in levels. So that meant, sort of like particles or bundles. So nothing happened until he hit a certain level, so he had enough light, and then the energy would jump. So he called them quanta, which just basically is a Latin word for bundles. And then Einstein went one step. Further, Huygens and Young proved that light has to be a wave. Planck and Einstein, on the other hand, argued that light has to be a particle, which aligns with our modern theory. As a result, depending on the situation, either idea can be correct. In this chapter, we will focus on treating light as waves and explore wave theories to support this notion. However, in the next chapter, we will examine the ideas of Einstein and Planck and consider light as particles. This approach reflects the current understanding of light. Of doing it, as we say, light is actually a particle that travels in a wave pattern. That is typically how we think of it now, and that's all.