Chapter 35c

# Chapter 35c - Chapter 35 Interference Interference in thin...

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Ch. 35: Interference - Part C 1 December 4, 2009 1 Chapter 35: Interference ! Interference in thin films – reflection and transmission. ! HITT question. ! Michelson’s Interferometer. ! Young’s Interference Experiment. ! Intensity in Double-Slit Interference.

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Ch. 35: Interference - Part C 2 December 4, 2009 2 The Index of Refraction ! The index of refraction n for any medium is defined as the ratio of the speed of light in vacuum and the speed of light in the medium. ν = c n (index of refraction) λ λ= n n λλ == = n n c ff ! The speed, wavelength and frequency of light in a medium with refraction index n are: ν= c n (wavelength) (speed) (frequency)
Ch. 35: Interference - Part C 3 December 4, 2009 3 Wavelength and the Index of Refraction ! The phase difference between two light waves can change if the waves travel through different materials having different indexes of refraction. ! The two light waves travel initially in air (n~1), and are initially in phase, then travel distance L in media with refraction indexes n 1 and n 2 , and then emerge in air. ! What is the phase difference in terms of wavelengths as the two waves emerge in air? n 2 >n 1 11 1 λλ == n LL Nn 22 2 n 21 2 1 2 1 () λλλ −= = LLL NN n n nn and

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Ch. 35: Interference - Part C 4 December 4, 2009 4 Reflection Phase Shifts ! Refraction at an interface does not cause phase shifts. ! Reflection at an interface can cause phase shifts depending on the indexes of reflection on the two sides of the interface. 0.5 wavelength Off higher index 0 Off lower index Reflection Phase Shift Reflection “Higher means half” rule.
Ch. 35: Interference - Part C 5 December 4, 2009 5 Interference from Thin Films ! The angle of incidence θ is ~0 o . ! The ray r 1 is reflected from the front (left) surface of the film. ! The ray r 2 is reflected from the back surface of the film. ! The region ac of the film will appear bright or dark to the observer viewing it almost perpendicularly, depending on whether the reflected rays r 1 and r 2 interfere constructively or destructively. ! Light waves (ray i ) are incident on a thin film of thickness L and index of refraction n 2 .

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Ch. 35: Interference - Part C 6 December 4, 2009 6 Reflections from a Thin Film in Air ! The angle of incidence θ is ~0 o . ! Ray r 1 is reflected from the front surface of the film, with a 0.5 λ n2 phase shift.
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Chapter 35c - Chapter 35 Interference Interference in thin...

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