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# 102 lect21_handout - Physics 102 Lecture 21 Diffraction...

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Physics 102: Lecture 21, Slide 1 Diffraction, Gratings, Resolving Power Physics 102: Lecture 21

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Physics 102: Lecture 21, Slide 2 Recall • Interference (at least 2 coherent waves) – Constructive (full wavelength difference) – Destructive (half wavelength difference) • Light (1 source, but different paths) – Young’s double slit – Thin films – Multiple slit – X-ray diffraction from crystal – Diffraction/single slit Last lecture Today’s lecture
Physics 102: Lecture 21, Slide 3 ACT: Double slit review θ d L dsin( θ ) θ y Which condition gives destructive interference? 1) dsin( θ ) = m λ 2) dsin( θ ) = (m+1/2) λ m = 0, ± 1, ± 2

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Physics 102: Lecture 21, Slide 4 θ d L θ Multiple Slits: (Diffraction Grating – N slits with spacing d) Assume screen is very far away (L>>d): d d Path length difference 1-2 = d sin θ Path length difference 1-3 = 2d sin θ Path length difference 1-4 = 3d sin θ =2λ =3λ 1 2 3 4 Constructive interference for all paths when dsin( θ ) = m λ m = 0, ± 1, ± 2
Physics 102: Lecture 21, Slide 5 θ d L θ Multiple Slits: (Diffraction Grating – N slits with spacing d) Assume screen is very far away (L>>d): d d 1 2 3 4 m = 0, ± 1, ± 2 Holds for arbitrary N d d d Constructive: dsin( θ ) = m λ Same condition as Young’s double slit!

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Physics 102: Lecture 21, Slide 6 θ d L θ d 1 2 3 Preflight 21.1 All 3 rays are interfering constructively at the point shown. If the intensity from ray 1 is I 0 , what is the combined intensity of all 3 rays? 1) I 0 2) 3 I 0 3) 9 I 0
Physics 102: Lecture 21, Slide 7 θ d

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102 lect21_handout - Physics 102 Lecture 21 Diffraction...

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