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S E C T I O N 37. 6 • Interference in Thin Films 1189 the two-slit interference pattern. Instead, we observe a dark fringe at point P+. From an electromagnetic wave undergoes a phase change of 180° upon reflection from a medium that has a higher index of refraction than the It is useful to draw an analogy between reflected light waves and the reflections of a transverse wave pulse on a stretched string (Section 16.4). The reflected pulse on a 37.6 Interference in Thin Films Interference effects are commonly observed in thin films, such as thin layers of oil on Consider a film of uniform thickness t and index of refraction n, as shown in Figure 37.17. Let us assume that the light rays traveling in air are nearly normal to the two • A wave traveling from a medium of index of refraction n 1 toward a medium of index of refraction n 2 undergoes a 180° phase change upon reflection when n 2 ' n 1 and undergoes no phase change if n 2 2 n 1 . • The wavelength of light & n in a medium whose index of refraction is n (see Section 35.5) is (37.14) where & is the wavelength of the light in free space. & n # & Rigid support String analogy 180 ° phase change n 1 n 1 n 2 n 2 < (a) Free support No phase change n 1 n 1 n 2 n 2 > (b) Figure 37.16 (a) For n 1 2 n 2 , a light ray traveling in medium 1 when reflected from the surface of medium 2 undergoes a 180° phase change. The same thing happens with a reflected pulse traveling along a string fixed at one end. (b) For n 1 ' n 2 , a light ray traveling in medium 1 undergoes no phase change when reflected from the surface of medium 2. The same is true of a reflected wave pulse on a string whose supported end is free to move. No phase change Air 180 ° phase change 1 2 A t Film Air B 3 4 Figure 37.17 Interference in light reflected from a thin film is due to a combination of rays 1 and 2 reflected from the upper and lower surfaces of the film. Rays 3 and 4 lead to interference effects for light transmitted through the film. |