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# LO9- Thin Film Interference

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Getting familiar with Thin Film Interference

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### LO9- Thin Film Interference

1. 1. THIN FILM INTERFERENCE LO9 Source: https://ﬁgures.boundless.com/16755/full/oil-20slick.jpe
2. 2. Produces colours of soap bubbles, oil ﬁlms, peacock feather and some type of butterﬂies.
3. 3. Net reﬂection — (2) and (6). If they are in PHASE, constructive interference will make net reﬂected ray relatively bright. Unless, destructive interference with no reﬂection. Source: Nelson Physics for Scientists and Engineers depend Extra distance travelled by light in coating
4. 4. MINDMAP Phase changes 𝜆n = 𝜆/n index of refraction Wavelength in a medium 1. 180o (hard reﬂection): medium with greater refraction index. 2. No change (soft): medium with smaller refraction index. 2t= m𝜆n/2 (constructive interference) —————————— t=(2m+1)𝜆n/4 (destructive interference) m=0,1,2,… thickness
5. 5. CONCEPT CHECK ☑ • n1= 1.50 , n2= 2.00 • In all cases, determine what happened to reﬂected light when its thickness is approaching zero. Source: Boston University
6. 6. SOLUTIONS • Case A: ( ✔ ) eliminated by destructive interference ( ) bright by constructive interference • Case B ( ✔ ) eliminated by destructive interference ( ) bright by constructive interference In both cases, light experienced half wavelength shift due to the inverted wave from the bottom of the surface, therefore create destructive interference.
7. 7. SOLUTIONS • Case C: ( ) eliminated by destructive interference ( ✔ ) bright by constructive interference • Case D: ( ) eliminated by destructive interference (✔ ) bright by constructive interference In case C, wave is inverted all the way therefore no shift in wavelenght. While in case D, neither wave is inverted.
8. 8. PROBLEM 1 An anti-reﬂective coat (n= 1.38) is used on a glass surface. a) Find the minimum thickness of the coating that can be used for blue light (480 nm). b) Find the next two possible thickness.
9. 9. SOLUTION • Visualize • Anti-reﬂective coating = destructive interference n=1.34 ⟷t
10. 10. SOLUTION a) 𝜆n = 𝜆/n 𝜆n = 480/1.34 = 358 nm 2t= 𝜆n/2 t = 358/4 = 89.5 nm b) Next two possible conditions, 2t= 3𝜆n/2 2t= 5𝜆n/2 t = 268.5 nm t= 447.5 nm Note:We need extra phase shift of 180o to create constructive interference therefore we need wall thickness of 𝜆n/4, 3𝜆n/4, 5𝜆n/4, and so on.