How do we KNOW light is a wave?


This episode was made possible by generous supporters on Patreon. Hey Crazies. I recently made a video about the origin of light. I said light was an electromagnetic wave, a disturbance in the electromagnetic field. But how do we actually know light is a wave? What if we’re wrong?! (Dramatic Music) Everybody calm down! We discovered light was a wave long before we knew what type of wave it was. To the timeline! In 1690, Christiaan Huygens published his “Treatise on Light” where he suggested light was a wave. Then, in 1704, Isaac Newton published “Opticks” where he suggested light was made of tiny little objects. The debate raged on for almost a century! It wasn’t until Thomas Young performed the first double-slit experiment in 1801, that we finally proved without a doubt that light is a wave. But what about photons? Shush! So, let’s get into the details of how this double-slit thing works. First, we need some kind of wall or screen in the back. It shines wherever light hits it. Now let’s say there’s a barrier with a couple openings. If light shines on the barrier, it’ll pass some of the light and block the rest. But those openings are way too big to test anything about the light itself. For convenience, we’re using visible light and the size of these waves is on the order of 100s of nanometers. That’s tenths of thousandths of millimeters. So tiny! We don’t have to make the openings that small, but any more than 1000 times bigger and the light won’t spread out. The width and spacing needs to be at least comparable to the wavelength, which is why we call them slits. With openings that thin, something unexpected happens. Instead of two skinny lines on the back wall, we get a pattern of thick bright lines with dark spaces in-between. Something we call an interference pattern. Interference is what happens when two or more waves overlap and combine. Light is a wave in the electromagnetic field. But what about photons? I said shush! Light is a wave in the electromagnetic field, but there’s only one electromagnetic field. It can only respond to the sum of everything that’s happening to it. That’s what we call superposition. Superposition is when two or more things cause a net response that’s just the sum of the responses that would have been caused by each thing individually. It happens all the time with waves. If you’ve got two waves like this, the sum of the two looks like this. When peaks line up with peaks, we call it constructive interference because it builds up an even bigger wave, which means brighter light. When peaks line up with troughs, we call it destructive interference because the waves cancel out, which means no light. If the waves cancel, where does the energy go? It just goes somewhere else in the field. The wave doesn’t cancel everywhere! Anyway, constructive and destructive interference aren’t the only two possibilities. There are many gradual levels of brightness in-between depending on how the waves meet each other. With our double-slit, we basically have two independent sources of light. They look like this from behind. Because those slits are narrow, the light from each slit will spread out. If each slit is sending out its own light waves, those waves are going to overlap and interfere with each other. In the middle of the wall, the waves have to travel the same distance. That makes their peaks meet up and we see a bright line. We’ll label that zero. Farther along the wall, we’ll find a place where peaks meet up with troughs. That’s the first dark line. So depending on how far each of these waves has to travel compared to the other, we can get bright lines or dark lines. The next bright line is where the light from one slit has to travel one wavelength farther than the light from the other slit. We label that one because the difference in distance is one wavelength. There’s another bright line at two wavelengths and another at three wavelengths and so on. Quick side note though. You get the best results with a single color of light. The equation for where to find the lines is wavelength dependent so, if you try this with white light, the pattern is harder to make out. End of side note. How many bright lines are there? That depends on the slit width and spacing. We can only get bright lines up to 90 degrees in either direction. Eventually, we run out of room. As I mentioned before though, there aren’t just bright lines and dark lines. There’s a gradual transition in brightness from one to the other. If you graph out the predicted intensities across the wall, they match the experiment perfectly. Science at its finest! What happens if we move the wall closer or farther away? It’ll change the size of the pattern, but not its shape. That wave interference is happening all throughout the space between the slits and the wall. If you could actually see the electromagnetic field, the places you’d find dark lines would be pretty obvious. But what about photons?! Photons are waves too, OK? Seriously! Their energy is measured indirectly through their frequency. Objects don’t have frequency. Waves do. When you do the same double-slit experiment with individual photons and you’ve got a screen that can keep a record of when light hits it, the same interference pattern builds up over time. No matter how you look at it, light is a wave. We know that because it can interfere with itself and that interference produces a pattern on a screen. Got any question about diffraction or interference? Please ask in the comments. If you were hoping for more about photons, don’t worry. I’ll be going into more detail next time. Thanks for liking and sharing this video. Don’t forget to subscribe if you’d like to keep up with us. Hit that notification bell if you’re really into what we do here. And until next time, remember, it’s OK to be a little crazy. The featured comment comes from Ado Sar who asked: If wavelength can vary from medium to medium, wouldn’t it be more accurate to define color with frequency? On a scale like ours, yes, we have to define color with frequency for that exact reason. But, on a microscopic level, light is never really inside a material. Anyway, thanks for watching!

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