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Physics 212 Lecture 28

Physics 212 Lecture 28. Optical Devices. Music. Who are you? Me Someone else i am imaginary ( ) Thought A superhero / supervillain in disguise E / c 2. Executive Summary – Mirrors & Lenses:. real inverted smaller. S > 2f. f > 0. real inverted bigger. 2f > S > f.

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Physics 212 Lecture 28

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  1. Physics 212 Lecture 28 Optical Devices

  2. Music • Who are you? • Me • Someone else • i am imaginary ( ) • Thought • A superhero / supervillain in disguise • E / c2

  3. Executive Summary – Mirrors & Lenses: real inverted smaller S > 2f f > 0 real inverted bigger 2f > S > f virtual upright bigger f > S > 0 f < 0 virtual upright smaller S > 0

  4. It’s always the same: You just have to keep the signs straight: Lens sign conventions Mirrors sign conventions S: positive if object is “upstream” of lens S’ : positive if image is “downstream” of lens f: positive if converging lens S: positive if object is “upstream” of mirror S’ : positive if image is “upstream” of mirror f: positive if converging mirror (concave) s’ is positive for a real image f is positive when it can produce a real image

  5. System of Lenses Image from first lens Becomes object for second lens • Trace rays through lenses, beginning with most upstream lens

  6. System of Lenses Object Distance is Negative !! Image from first lens Becomes object for second lens • Virtual Objects are Possible !!

  7. A parallel laser beam of width w1 is incident on a two lens system as shown below. Checkpoint 3 Each lens is converging. The second lens has a larger focal length than the first (f2 > f1). What does the beam look like when it emerges from the second lens? A. The beam is converging B. The beam is diverging C. The beam is parallel to the axis with a width < w1D. The beam is parallel to the axis with a width = w1E. The beam is parallel to the axis with a width > w1 A B C D E “Not sure about the width, but with larger f2 it seems width would be greater too :/”

  8. A parallel laser beam of width w1 is incident on a two lens system as shown below. Checkpoint 3 Each lens is converging. The second lens has a larger focal length than the first (f2 > f1). What does the beam look like when it emerges from the second lens? A. The beam is converging B. The beam is diverging C. The beam is parallel to the axis with a width < w1D. The beam is parallel to the axis with a width = w1E. The beam is parallel to the axis with a width > w1 A B C D E • Parallel rays are transmitted and pass through focal point (f1) • Those rays also pass through focal point of second lens (f2) and therefore are transmitted parallel to the axis. • f2 > f1 implies that the width > w1

  9. Multiple Lenses Exercises real image s’ > 0 inverted image M < 0 Two converging lenses are set up as shown. The focal length of each lens is 47 cm. The object is a light bulb located 70 cm in front of the first lens. s1=70 cm f=47 cm • What is the nature of the image from the first lens alone? • REAL (B) REAL (C) VIRTUAL (D) VIRTUAL • UPRIGHT INVERTED UPRIGHT INVERTED PICTURES EQUATIONS Draw Rays as above s > f

  10. Multiple Lenses Exercises s2 = -0.57 Image of first lens is a REAL object for the second lens OR s2 = +0.57 Two converging lenses are set up as shown. The focal length of each lens is 47 cm. The object is a light bulb located 70 cm in front of the first lens. Lens separation = 2 m s1 = 70 cm f = 47 cm s1’ = 1.43 m • What is the object distance s2 for lens 2? • s2 = -1.43 m (B) s2 = +1.43 m (C) s2 = -0.57 m (D) s2 = +0.57 m (E) s2 = +2.7 m THE OBJECT FOR THE SECOND LENS IS THE IMAGE OF THE FIRST LENS

  11. Multiple Lenses Exercises s2’ > 0 real image s2 > f RESULTS s2’ = 2.69 m M = 9.6 M2 < 0 M = M1M2 > 0 upright image Two converging lenses are set up as shown. The focal length of each lens is 47 cm. The object is a light bulb located 70 cm in front of the first lens. Lens separation = 2 m s1’ = 1.43 m s1=70 cm f=47 cm s2 = 0.57 m • What is the nature of the FINAL image in terms of the ORIGINAL object? • REAL (B) REAL (C) VIRTUAL (D) VIRTUAL • UPRIGHT INVERTED UPRIGHT INVERTED PICTURES EQUATIONS Draw Rays as above

  12. Multiple Lenses Exercises s2 = -0.48 Image of first lens is a VIRTUAL object for the second lens OR s2 = +0.48 Suppose we now decrease the initial object distance to 58 cm. Applying the lens equation, we find s1’ = 2.48m Lens separation = 2 m s1 = 58 cm f = 47 cm s1’ = 2.48 m • What is the object distance s2 for lens 2? • s2 = -0.48 m (B) s2 = +0.48 m (C) s2 = -2.48 m (D) s2 = +2.48 m (E) s2 = +2.58 m THE OBJECT FOR THE SECOND LENS IS THE IMAGE OF THE FIRST LENS

  13. Multiple Lenses Exercises s2’ > 0 real image s2 < 0 RESULTS s2’ = 0.24 m M = -2.1 M2 > 0 Suppose we now decrease the initial object distance to 58 cm. Applying the lens equation, we find s1’ = 2.48m Lens separation = 2 m s1=58 cm f=47 cm s1’ = 2.48 m s2 = -0.48 m • What is the nature of the FINAL image in terms of the ORIGINAL object? • REAL (B) REAL (C) VIRTUAL (D) VIRTUAL • UPRIGHT INVERTED UPRIGHT INVERTED PICTURES EQUATIONS Draw Rays as above M = M1M2 < 0 inverted image

  14. Normal Eye

  15. Checkpoint 2 A person with normal vision (near point 28 cm) is standing in front of a plane mirror. What is the closest distance to the mirror the person can stand and still see herself in focus? A. 14 cm B. 28 cm C. 56 cm

  16. Checkpoint 2 qr qi d d A person with normal vision (near point 28 cm) is standing in front of a plane mirror. What is the closest distance to the mirror the person can stand and still see herself in focus? A. 14 cm B. 28 cm C. 56 cm The image is formed an equal distance BEHIND the mirror Therefore, if you stand a distance = ½ of your near point, the distance to the image will be the near point distance.

  17. Far-sighted Converging Lens creates virtual image at person’s near point

  18. Near-sighted Fix with diverging lens that creates virtual image at far point.

  19. Checkpoint 1 Two people who wear glasses are camping. One of them is nearsighted and the other is farsighted. Which person’s glasses will be useful in starting a fire with the sun’s rays? A. The farsighted person’s glasses B. The nearsighted person’s glasses “My glasses have never started a fire and I am nearsighted. That has to count for something. Also, magnifying glasses seem like it would be closer to a farsighted person's glasses and tv characters are always using them” “...you'd let a nearsighted guy without his glasses on attempt to start a fire?.”

  20. Checkpoint 1 Two people who wear glasses are camping. One of them is nearsighted and the other is farsighted. Which person’s glasses will be useful in starting a fire with the sun’s rays? A. The farsighted person’s glasses B. The nearsighted person’s glasses Farsighted = Converging Lens Only Converging Lens can produce a REAL IMAGE !!

  21. The Large Synoptic Survey Telescope

  22. How to make a big telescope mirror Melt it & spin it 52,000 lbs of borosilicate glass when filled

  23. A big telescope mirror

  24. What to do with a big telescope mirror: A system of lenses and mirrors

  25. Course Review on Tuesday • The most popular topics • Electric Fields/Gauss’ Law/Potential • Faraday’s Law • RC/RL Circuits • AC Circuits • Selected old exam problems • Notes on the End Game • Final Exam: 50questions uniformly covering topics in course • should have already chosen “combined”/”conflict” • Don’t forget to check room assignments!! • About 40 students still have not registered their iclicker • Do it now or accept all zeroes for lectures • Course Grade: See “Course Description” link on homepage

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