27B thin film, interference

# 27B thin film, interference - Outline Physics 1C Lecture...

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5/18/2011 1 Physics 1C Lecture 27B "You cannot trust your eyes if your imagination is out of focus." --Mark Twain Outline Thin film interference Single slit diffraction Resolution Interference in Thin Films Interference in soap bubbles. The colors are due to interference between light rays reflected from the front and back surfaces of the thin film of soap making the bubble. The color depends on the thickness of the film. Black appears where the film is thinnest. Red appears where the film is thickest. Interference in Thin Films With light incident on a thin film we have to examine two light rays that follow the same path: Ray 1 (in air) reflects off the film surface and undergoes a phase change of 180 o compared to the incident ray. Ray 2 (in air) refracts at the film surface and then reflects off of Surface B (film to air) with no phase change compared to the incident wave. Interference in Thin Films In addition, Ray 2 travels a distance t down and and a distance t up (for a total distance 2 t ). Since the extra distance that Ray 2 travels is in the film medium, its wavelength will be different then in air. The wavelength of light, λ n , in a medium with index of refraction n is: n n where λ is wavelength of light in a vacuum (air?). Interference in Thin Films In order for constructive interference to occur, the two rays must be in phase. This means that the difference in path length and phase must combine to m ( λ / n ) , where m = 0, ±1, ±2. .. The difference in path is: 2 t and the difference in phase is: Constructive Interference for 1 phase change. Combining all of these gives us: 1 2 n   2 t 1 2 n m n 2 nt m 1 2

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5/18/2011 2 Interference in Thin Films With light incident on a thin film with another thin film of higher index of refraction below the first: Ray 1 (in air) reflects off the film surface and undergoes a phase change of 180 o compared to the incident ray. Ray 2
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27B thin film, interference - Outline Physics 1C Lecture...

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