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PH 103 Dr. Cecilia Vogel Lecture 15

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Review Outline Refraction index of refraction, law of refraction Lenses focal length images thin lens equation ray diagrams

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Lenses Two types converging center thicker sometimes called convex or positive diverging edges thicker sometimes called concave or negative

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Converging Lens Light comes in perpendicular to lens. What happens to it, if it passes through : top of lens bent downward center straight thru ( =0) bottom bent upward CONVERGE Demo of converging lens f

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Diverging Lens Light comes in perpendicular to lens. What happens to it, if it passes through : top of lens bent upward center straight thru ( =0) bottom bent downward DIVERGE Demo of diverging lens f (negative)

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Focal Point and Images Is focal point the point where light is in focus? Only if rays come in parallel (from very far) Image point is point where light is in focus Position of image point depends on position of object type of lens focal length of lens Thin lens equation

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Ray Diagrams Step-by-step for converging lens: 0. Determine an appropriate scale. 1. Place lens, and draw an axis thru center of lens and perpendicular to it. 2. Draw an object (arrow) the object distance from lens. Make base of arrow on axis. 3. Place F one focal length behind lens on axis. 4. One ray from tip of arrow goes parallel to the axis until it hits the lens, then it goes out through F. (“P-ray”)

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5. Second ray from tip of object goes through center of lens, and goes straight through. (“M-ray”) 6. Image of the tip of the arrow is where both outgoing rays seem to come from. 7. Optional check: place F’ one focal length in front of lens. 8. Optional check: Third ray through F’ continues until it hits lens, then goes out parallel to axis. (“F-ray”)

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Step-by-step for diverging lens: 0. Determine an appropriate scale. 1. Place lens, and draw an axis thru center of lens and perpendicular to it. 2. Draw an object (arrow) the object distance from lens. Make base of arrow on axis. 3. Place F one focal length in front of lens. 4. One ray from tip of object goes parallel to the axis until it hits the lens, then goes out as if it had come from F (that is, the outgoing ray is inline with F, but not though it). (“P-ray”) Ray Diagrams

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5. Second ray from tip of object goes through center of lens, and goes straight through. (“M-ray”) 6. Image of the tip of the arrow is where both outgoing rays seem to come from. 7. Optional check: place F’ one focal length behind lens. 8. Optional check: Third ray headed toward F’ continues til it hits lens, then goes out parallel to axis. (“F-ray”)

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Examples on board f = +25 cm, d o = 75 cm f = +25 cm, do = 10 cm f = -30 cm, d o = 15 cm Java applet Java applet Ray Diagrams

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Check examples on board by calculation f = +25 cm, d o = 75 cm f = +25 cm, do = 10 cm f = -30 cm, d o = 15 cm Ray Diagrams

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Lens Equations and Sign Conventions dodo positive if in front of lens (negative if behind) didi positive if behind negative if in front f positive if converging negative if diverging

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Lens Equations and Sign Conventions h o positive if up negative if down h i positive if up negative if down M positive if same side up negative inverted

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