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**Unformatted text preview: **Class 18 Thin Films and Diffraction Gratings Physics 106 Fall 2011 Press CTRL-L to view as a slide show. Last Time I Mirror Problems I Images Formed by Refraction I Thin Lenses, Ray Diagrams, and the Lens Equation I Aberrations Learning Outcomes Today we will discuss: I Double Slit Interference I Phase Shifts and Reflection I Thin Film Interference Double Slit Interference Double Slit Equations The basic idea I The distance from one slit to the screen is L 1 . I The distance from the other slit to the screen is L 2 . I If Δ L = L 2- L 1 equals 0, 1, 2 ... wavelengths, the interference is constructive. I If Δ L = L 2- L 1 equals 1/2, 3/2, 5/2 ... wavelengths, the interference is destructive. Double Slit Equations The variables: I θ us the viewing angle with θ = 0 straight ahead. I d is the separation distance between the slits. I λ is the wavelength. I The path difference Δ L = d sin θ I m is the order number m = , ± 1 , ± 2 ,... Order Number in Double Slit Interference I m =- 2 I m =- 1 I m = I m = + 1 I m = + 2 With one slit, a bright band would appearstraight ahead. The m = 0 band is straight ahead. Other bands are above and below. Double Slit Equations The equations with θ I Constructive Interference: m λ = d sin θ I Destructive Interference: ( m + 1 / 2 ) λ = d sin θ Double Slit Equations The equations with y I L is the distance from the slits to the screen, y is the distance from the center of the middle bright spot. I Constructive Interference: m λ = d y L I Destructive Interference: ( m + 1 / 2 ) λ = d y L Double Slit Problem I In our classroom, laser light with wavelength 632.8 nm passes through double slits separated by 1/4 mm. The pattern on the wall has dark bands separated by 1.1 cm. What is the distance from the laser to the screen? Double Slit Problem I In our classroom, laser light with wavelength 632.8 nm passes through double slits separated by 1/4 mm. The pattern on the wall has dark bands separated by 1.1 cm....

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