 When you pay close attention, you can see some pretty cool stuff that you might otherwise miss, like these soap bubbles. Notice how you can see a bunch of different colors. What about the layer of oil on the floor? See the rainbow of colors? Even the beautiful vibran colors on this butterfly. It's called, it's called a blue, a blue morpho butterfly and turns out all of these colors are due to thin film interference. And in this video, we will understand how exactly does thin film interference leads to such wonderful colors. And we will also see why a water drop or a window paint does not show these colors. Alright, so let's begin. We will work on the example of a soap bubble, but before that, let's break down this phrase thin film interference. Over here thin film means just a thin layer, a thin layer of material whose thickness, whose thickness can range from nanometers to micrometers. So it is extremely, extremely thin and interference, interference is when two waves meet or overlap while traveling along the same medium. So we know that sometimes that overlap can lead to constructive interference. That is when the crests or the peaks line up with the peaks and the drops line up with the troughs. We can also say that these two waves are in-phase. And sometimes when the waves are out of phase, that is when the crests or the peaks line up with the troughs and the troughs line up with the peaks, this sort of situation leads to destructive interference. Alright, now that we have some idea of what each of these words mean, let's bring back the soap bubble. So here it is. The wall of a soap bubble is just a thin film of water. This is water that is between two layers of soap. Now let's zoom, let's zoom into this. So we have zoomed right into one small tiny microscopic section of the soap bubble such that the, such that the layers say almost seem like straight lines. So this is the thin film. There's water in between these two white layers of soap, the white lives, the white layers of soap. Now light from the sun, which has colors of different wavelengths. So red has the longest wavelength and violet has the shortest wavelength. There might be a color missing over here. I think that's blue. Anyway, this light is incident on the top surface of the thin film and because there is a change in the medium, it undergoes a refraction and then coming out of the bottom surface of the film, it gets refracted again. But turns out some of the light reflects back from the first surface. Some of the light is refracted and some of the light is reflected off the top surface. And some of the light is reflected, some of the light is reflected from the bottom surface. After undergoing reflection at this point, it undergoes refraction and it comes out parallel to the first ray. Now the key here is that wave number one, wave number one and wave number two, they travel different amount of distances. The light wave reflected from the bottom surface, it travels this much extra distance. So there is a path difference between these two waves, between waves one and two. Now these two waves, one and two, they are quite close to each other, so they can meet and overlap, they can interfere and our eye that can be that can be on the top. This is the entire eyeball. The eye can experience interference because these two light waves are gonna hit the eye, they will converge on the retina and this can lead to either constructive or destructive interference depending on whatever this path difference is. Now as there is a path difference, waves one and two could either be in phase or out of phase. If they are in phase then their peaks would match with the peaks and troughs would match with the troughs. In this case, they will constructively interfere and we will see that color. So if the green light constructively interferes, we will see a green color film. But there could also be a possibility that as a result of this path difference, waves one and two are out of phase and the peaks are matching with the troughs and the troughs with the peaks. In this case, the waves will destructively interfere and we will not see that color in that section of the soap bubble. This thickness of the soap film, it determines the path difference and if you change the thickness, you can change the path difference which could lead to different colors interfering constructively and destructively. That is one factor on which whether the lights are interfering constructively or destructively depend upon. The second is also, you could imagine the angle of this incident light. If this angle changes, the path difference will also change. If this incident angle was more, that is if the incident ray of light came from this direction, then the path difference would also be more in that case. As the light will get refracted and then go back, so it will be traveling more, more distance. And maybe because of that path difference, we might see constructive interference of some different color. Maybe not green. So the angle of incidence and the thickness of the soap film can determine which light will interfere constructively and destructively. And this thickness, it varies or changes all over the soap bubble. So different wavelengths of light are interfering constructively or destructively at different parts of the bubble. And that's why we see, we see different colors in different areas of the bubble. And that's why we see these really cool rainbow colors. What about, what about the butterfly though? Now the blue color here is not due to a pigment color of the wings. In fact, the pigment color of the wings of a blue morpho butterfly is brown. It's not blue. But the color is due to, due to thin film interference, but more specifically it is due to the structure of the wings. If we zoom into the wings, the wings of the butterfly have these tiny microscopic structures with air gaps in between. And they act as multi-layer thin films. These films are just the right thickness for the blue wavelength of light to constructively interfere. And for almost every other wavelength of light to destructively interfere. And that's why we see this vibrant blue color on this butterfly. Let's talk about why we don't see such patterns with a water drop or a window pane. For this, we take the situation of a vertical soap bubble. Since, since this film is vertical, it is thinner at the top and thicker at the bottom due to gravity's pull. So the cross section of the film can somewhat look like this. Thin at the top and it gradually gets thicker and thicker near the bottom. We see that the fringe pattern is clear and distinct in this, in this section of the bubble. And as the thickness increases, the fringes start getting close to each other until we are barely able to separate them. And we end up seeing white at the bottom or washed out white, more or less. Why does this happen? Let's, let's look at the thin film again. Now this yellow light can interfere constructively, but to get a different color, we can change the thickness or the angle. We're not changing the thickness right now. Let's, let's just change the angle and see what happens. If we, if we change the incident angle, if we change this angle slightly, then the ray of light would be coming from here. And it will undergo refraction and then reflection, but, but there won't be much change in the part difference, right? Because the thickness here is at the same order as the wavelengths of the light that are incident on it. So if we change the angle slightly, just slight change in the angle, we are giving some extra part difference for it to move through the film, but not enough for, let's say, a different color wavelength of light to interfere constructively. On, on slight change of the angle, we will still see yellow light, but maybe not as bright, though we will still see yellow light. If we change the angle enough, maybe like this, so that now you see how much extra distance the light is traveling inside the film. You're giving all this extra part difference. The total part difference would still remain as this total distance, but now it is traveling some more distance. There is some more part difference. There is an extra part difference. Maybe we have changed it enough so that a new color with a different wavelength is now in phase due to the extra part difference. Then we will start seeing that color. Now we will start seeing orange color. This tells us that the fringe patterns here will take some space for themselves. You need to change the angle enough so that you see a fringe pattern for a different color. Therefore, in this section where the, where the film is relatively thinner, we can easily separate out the fringes. We can easily see where the green is ending, where the red is beginning, where the blue is ending, where the yellow is beginning. Our eyes can make out where one color ends, where the other begins. But when the thickness is increased, let's see what happens then. So here we have a really thick film. Now maybe yellow light is interfering constructively to begin with and we change the angle slightly. So again, the normal, here it is. Now we are changing this angle, this insred angle just slightly, just a slight change. And let's see what happens then. So maybe the new rays, the new rays might look like this. Let me make this transparent slightly so that we can see this. All right. So just a slight change in angle and see how much extra path the new light has to travel in the film. We can, we can see that these two sides, these two sides are considerably larger than, than these two sides. Just a slight change in the angle and there is so much extra part difference. Maybe one entire wavelength, who knows, could be two wavelengths. But the idea is that now the light has to travel much greater distance, even with a very slight change in the angle. This amount of extra distance traveled along with the entire part difference could be enough for a new color of wavelength to constructively interfere. And that is what happens. Just a slight change of angle and now orange light constructively interferes. This does hint towards the idea that when you increase the thickness, the fringes will appear very close to each other. The bright fringe of a certain light will be very, very close to the bright fringe of a different light. There is a certain thickness of film when the maxima, when the maxima of one light is on the top of the minima of the other light. When this happens, our eyes are just, just able to see these two colors separately. If you then further increase the thickness, then we will not be able to separate out the colors. All the colors start being on top of each other and we start seeing a whiteish or washed out white sort of a situation here. And turns out that the thickness of a water dropper or a window pane is so large that we are not able to figure out where one color ends, where the other would begin. They are just on top of each other and we do not see any, any fringe pattern whatsoever.