Because the exact thickness of the film varies across the piece of ham, which is illuminated by white light, different wavelengths produce bright fringes at different locations, resulting in … For instance, you must have noticed some rainbow-coloured reflection in a pool of water on the side of the road or in a bubble. Note that you have to be very careful to account for whether a phase shift occurs at an interface where reflection is taking place. In this case, there is no phase change due to reflection at B and C because in both cases light is travelling from denser to rarer medium. Thin-film interference is a common occurrence that happens in our everyday lives. It occurs mostly when light is reflected several times off (interference) the sides of thin transparent material. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. Consider the case of a thin film of oil floating on water. The phase difference is the difference in phase angle between two waves. Thin-film interference is the phenomenon that is a result of lightwave being reflected off two surfaces that are at a distance comparable to its wavelength. This must account for any phase shift introduced by a reflection off a higher-n material, as well as for the extra distance traveled by the wave traveling down and back through the film. Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. r = Angle of refraction, When we take a soap bubble, light waves travel through the air and hits the soap film. will undergo a 180° shift. Thin-film interference thus depends on film thickness, the wavelength of light, and the refractive indices. The shift occurs mainly because of the lower refractive index of the air than that of the film. Refractive indices of the material on either side of the film. Meanwhile, the type of interference that can take place is dependent upon these factors. On the other hand, a 180° phase shift will be introduced in the reflected wave. A soap film of refractive index 1.33 is illuminated by the light of wavelength at an angle of 45 degrees. For non-reflective coatings, then, the minimum film thickness required is: where n is the index of refraction of the coating material. Soft reflection: Reflects off a medium with a lower refraction index, as a result, no phase shift is observed. In the case of 1/4 wavelength, the wave reflected off the top surface is shifted by 1/2 a wavelength by the reflection. Maximum interference in the reflected light, in case of thin-film interference, expressed as, μ = Refractive index of the film relative to the surrounding, t = Thickness of film and The surface of the ham being moist means there is a thin layer of fluid, resulting in thin-film interference. When the second medium is a thin film, there are two reflections occurring close together at the top and bottom b… In some cases, the minimum film thickness required for constructive interference is a quarter of the wavelength; in other cases, the minimum film thickness must be half a wavelength for constructive interference to take place. the light traveling from the air, through the oil, reflecting off the bottom surface, traveling back through the oil and out into the air again. On the other hand, if the film thickness is 1/2 wavelength, the first wave gets a 1/2 wavelength shift and the other gets a wavelength shift; these waves would cancel each other out. The interference can be constructive or destructive depending on the phase difference between the two reflected light waves, resulting in the increase or decrease in the brightness of the reflected light. Hard reflection: Reflects off a medium with a higher refraction index, as a result, a phase shift is observed of λ/2. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. Note that a shift by 180° is equivalent to the wave traveling a distance of half a wavelength. The relation between phase difference and path difference is that they are directly proportional to each other. During this, the light reaches the boundary between two media and part of it gets reflected and some part gets transmitted. Monochromatic source results in interference patterns that appear as light or dark bands. When light waves that reflect off the top and bottom surfaces interfere with one another we see different coloured patterns. This observable effect is known as thin-film interference. Thus, there are two waves emerging from a thin film – one wave reflected off the top surface of the film and the other reflected off the bottom surface. Such coatings are commonly found on some camera lenses or binocular lenses, and often have a bluish tint. We shall study more in-depth about this concept below. A thin film of soap solution is illuminated by white light at an angle of incidence i = sin-1(4/5). An important consideration in determining thin-film interference is also by knowing the source of the light. The film thickness, for constructive interference in the example above, has to be 1/4 (or 3/4 or 5/4 or ...) of the wavelength of the light in the oil. In addition to the path length difference, there can be a phase change. Thin-film interference can take place if these two light waves interfere constructively: An important consideration in determining whether these waves interfere constructively or destructively is the fact that whenever light reflects off a surface of higher index of refraction, a 180° phase shift in the wave is introduced. The coating is put over glass, and the coating material generally has an index of refraction less than that of glass. When the thickness is a multiple of a half-wavelength of the light, the two reflected … In that case, then, both reflected waves have a 180° phase shift, and a film thickness of 1/4 wavelength (in the film) would produce a net shift of 1/2 wavelength, resulting in cancellation. Destructive interference occurs when the thickness of the oil film is 1/2 wavelength, 1 wavelength, 3/2 wavelength, etc. In the reflected light, two dark consecutive overlapping fringes are observed corresponding to wavelengths 6.1 x 10-7 m and 6.0 x 10-7 m. The refractive index for soap solution is 4/3. 1. From the geometry of the triangles AEB, AFB, ABC, and ACF, In triangle ACD, Sin i = AD / AC –––––– (i), In triangle AGC, Sin r = GC / AC –––––– (ii), Now, Path difference = µ (AB + BC) – AD = µ (AB + BC) – µGC = µ (AB + BC – GC), Path difference = µ (BF + BC – GC) [because AB=BF], FG = 2t cos r [because AF = AE +EF = t + t], where t is the thickness of the thin film. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Thin film interference occurs between the light reflected from the top and bottom surfaces of a film. If we look at the refractive index of air it is 1(nair = 1) whereas the film will have an index larger than 1 (nfilm > 1).
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