Now, make a prediction - when you switch to green light, will the smallest non-zero film thickness that gives constructive interference for green light be larger than, smaller than, or equal to the thickness you found for the red light. If only one of the reflections results in an inversion, however, the effective path-length difference is 2t plus or minus (it doesn't really matter which) half a wavelength. Description. If a thin oil film is floating on the water, you will see a beautiful pattern appear on the oil film. Thin Film interference – GeoGebra Materials. For instance, if you have both red and blue incident light, the incident light would look purple to you, because it is actually red and blue mixed together. It is just a little more complicated than this, however - there are two more ideas that we need to consider. The two primary reflected waves interfere, sometimes constructively. If so, how can you explain this. Based on our previous understanding of interference, we might expect that if this path-length difference was equal to an integer number of wavelengths, we would see constructive interference, and if the path-length difference was an integer number of wavelengths, we would see destructive interference. What similarities and differences do you observe for your two sets of observations? The two primary reflected waves interfere, sometimes constructively. In addition to the path length difference, there can be a phase change. Note that, in the simulation, the incident wave is shown on top. With the initial settings for the indices of refraction of the various layers, vary the film thickness to determine which film thicknesses result in constructive interference for the reflected light, and which result in destructive interference for the reflected light. Repeat the observations you made in steps 1 and 2 above. At the left of the simulation, you can see some colored boxes representing the color of the incident light, the reflected light, and the transmitted light. Justify your prediction, and then try it to see if you were correct. Thus, in our thin-film situation, if both reflections result in an inversion, or neither one does, the 2t path-length difference we derived above is all we need to consider. This is a simulation of thin-film interference. The wave that reflects off the back surface of the film is moved even farther below. To satisfy the interference conditions, we need to align the wave that goes down and back in the film with the wave that bounces off the top of the film. With this simulation, you can explore thin-film interference. When light traveling in one medium is incident on a thin film of material that is in contact with another medium, some light reflects off the top (or front) surface of the film, and some light goes through the film, reflects off the bottom (or back) surface of the film, and emerges back into the original medium. Rainbow colors are common in soap bubbles. In thin-film interference, light waves reflect of the front and back surfaces of a transparent thin-film. Thin-Film Interference. Now, adjust the index of refraction of medium 1 so that it is larger than that of medium 2. The difference from the rainbow in the sky is that the bands of color tend to repeat. With this simulation, you can explore thin-film interference. This is because the two… How can you explain this? When light traveling in one medium is incident on a thin film of material that is in contact with another medium, some light reflects off the front surface of the film, and some light goes through the film, reflects off the back surface of the film, and emerges back into the original medium. Second, we have to account for the fact that when light reflects from a higher-n medium, it gets inverted (there is no inversion when light refllects from a lower-n medium). Express these thicknesses in terms of the wavelength of the red light in the film. Part of the implementation of the simulation includes reversing the order of the layers when we see backfacing geometry, such that the substrate becomes the medium and the medium becomes the substrate in the calculations. Look at the interference that occurs between the two reflected waves traveling towards the left in the first medium. These two reflected waves then interfere with one another. A thin film of air between a plano-convex lens and a glass flat. Thin Film interference. One wave is reflected from the surface, and the other is reflected from the inside. With this purple (red and blue, that is) incident light, can you find a film thickness that produces blue reflected light and red transmitted light? With this simulation, you can explore thin-film interference. If the film thickness is t, then the second wave travels an extra distance of 2t compared to the first wave. Use the sliders or input boxes to adjust the index of refraction of the material in front of the thin film, the thin film, and the material behind the thin film, as well as the thickness of the thin film and the wavelength of the incoming light. Inverting a sine wave is equivalent to simply moving the wave half a wavelength. Once we've determined the effective path-length difference between the two waves, we can set that equal to the appropriate interference condition. Start with only the red light source on, and showing the interference for red light. From the wavelength and the radii of the interference rings, we determine the radius of curvature of the lens. oPhysics: Interactive Physics Simulations. Find the smallest non-zero film thickness that gives constructive interference for the reflected light when the light is red. 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