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# Lens unit 2010 5

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### Lens unit 2010 5

1. 1. Accelerated Physics Lens Unit 2010 Lenses Activity Homework 5/13 Thursday Go over test Snell’s Law / Intro to Lenses Go over homework Phet Lens WS (due MON) 5/14 Friday Exploring Lenses Activity Lens Equation WS (Front Side) To discuss images Phet Lens WS To learn lens equation 5/17 Monday To go over homework Ray diagrams for convex Learn convex ray diagrams Lens Equation WS (Back side) Start lens diagrams 5/18 Tuesday Learn concave ray diagrams Ray diagrams for concave To start lens lab Eye article (due Thursday) 5/19 Wednesday To complete lens lab Finish lab / follow up questions Read eye article 5/20 Thursday Quiz over reading Finish post-lab WS Learn about vision / correcting vision problems Lens Graphing Practice Post-lab WS 5/21 Friday Real lens activity Read and take notes over section (Assembly (determine vision problems) 25.2-25.3 (focus on concepts NOT Schedule) Lens applications equations) 5/24 Monday Finish lens applications Review sheet Review for the test 5/25 Tuesday Test over lenses Study for final 5/26 Wednesday Go over test Review for the Final Review for the final 5/27 Thursday Review for the final Review for the final 5/28 Friday First Day Of Finals Review for finals Periods 1, 2, and 4 5/31 Monday Memorial Day – No School Review for finals 6/1 Tuesday Second Day of Finals Review for finals Periods 3, 7, 5 6/2 Wednesday Last day of Finals Enjoy the Summer! Periods 8, 6 1
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3. 3. Lens Phet Physics Simulations For all of these demos go to the following website http://phet.colorado.edu, click on go to the simulations and click on “Light and Radiation”. Geometric Optics 1. Circle the best answer for each of the following statements: (Adjust the sliders up top to verify your answers) (a) A lens with a higher refractive index (index of refraction) will bend the more/ less. (b) A more/ less curved lens will bend the light more. (c) A more/ less curved lens will have a shorter focal length. (d) A bigger/ smaller diameter lens will allow more light in. 2. Use the sliders up top and make the refractive index 1.5, curvature radius 0.3 m, and the diameter 1.3 m. Select “virtual image”, “ruler”, and “principle rays”. Select “change object” until you get an arrow. Move the ruler and record the focal length. 3. Move the object so do = 20 cm, 30 cm, 40 cm, 60 cm, 80 cm. Be sure that the object is above the principle axis. do classify do measure classify di real image size (do < F, di (di = 2F, vs. (bigger, (use ruler) virtual smaller, etc) etc) same) 20 cm 30 cm 40 cm 60 cm 80 cm 4. Use the lens equation your measured F and the given value for do to calculate what di should have been. How do these values compare to what you measured? 3
4. 4. Reading: What Is A Lens? Have you ever snapped a photo or looked through a microscope? Of your answer is yes, then you have used an optical device. There are many kinds of optical devices. Cameras and microscopes are two examples. Others are projectors, binoculars, telescopes, and even eyeglasses. Every optical device is different. But they all have one thing in common. Each one has at least one lens. What is a lens? A lens is a transparent substance that bends or refracts light in a definite way. Most lenses are made of glass. Many lenses are made of plastic. Most lenses have one or two curved surfaces. There are two main types of lenss: convex [kon-VEKS] and concave [kon-KAVE]. • A convex lens is thicker at the center than at the edge. It magnifies or makes things look bigger. A convex lens converges, or brings together, light rays. The point where the light rays meet is called the focal [FOE-kul] point. Light that passes through a convex lens can be focused on a screen or other surface. This forms an image of the object that gave the light. Convex lenses are used in projectors and cameras. • A concave lens is thinner at the center than at the edge. It minifies or makes things look smaller. A concave lens spreads out light rays. They cannot form an image on a screen. Concave lenses are often used together with convex lenses. They help the convex lens give sharper images. Most eyeglass lenses have a combination of concave and convex curves 4
5. 5. UNDERSTANDING LENSES Six lenses are shown in Figure A. Study them. Then answer the questions by writing the correct letter. What you need to know: Plano means “plane” or “flat.” Which lens or lenses … 1. Are thicker at the center than at the edge? 9. Magnify? _____ _____ 10. Minify? _____ 2. Are thinner at the center than at the edge? 11. Refract light? _____ _____ 12. Converge light? _____ 3. Are concave? _____ 13. Diverge light? _____ 4. Are convex? _____ 14. Can form an image on a screen? _____ 5. Are plano convex? _____ 15. Cannot form an image ona screen? 6. Are plano concave? _____ _____ 7. Is double concave? _____ 16. Are most important for projectors and 8. Is double convex? _____ cameras? _____ Now look at figure B. 17. a. Figure B shows a [concave / convex] lens. b. it [converges / diverges] light rays. 18. What do we call the point where light rays converge? ___________________ 19. What do we call the distance between a lens and its focal point? _________________ 5
6. 6. ABOUT FOCAL LENGTH Different lenses have different focal lengths. Focal lengths depend upon the strength of a lens. o The stronger the lens, the shorter the focal length. o The weaker the lens, the longer the focal length. A strong lens has a deeper curve than a weak lens. Two converging lenses are shown in Figure C. Study the figure. Then answer the questions by writing the correct letter. Which lens… 1. is more curved? _____ 6. Refracts light more? _____ 2. Is less curved? _____ 7. Has the shorter focal length? _____ 3. Is stronger? _____ 8. Has the longer focal length? _____ 4. Is weaker? _____ 9. Magnifies more? _____ 5. Refracts light less? _____ 10. Magnifies less? _____ 6
7. 7. Convex Ray Diagrams The Three Rays: 1. 2. 3. One Special Case for Convex Ray Diagrams 7
8. 8. Concave Ray Diagrams The Three Rays: 1. 2. 3. All Ray Diagrams for Concave Lenses are the Same, Try One More: 8
9. 9. Convex Ray Diagrams Directions: 1. Find the image AND state if it is Real or Virtual 2. Use the lens equation and compare the computed di to the drawn di. (You will need a ruler) 9
10. 10. Concave Ray Diagrams Directions: 1. Find the image AND state if it is Real or Virtual 2. Use the lens equation and compare the computed di to the drawn di. (You will need a ruler) 10
11. 11. Lens Equation Worksheeet Example: A car is 2 m in front of a 1 m focal length lens. Where will the image be located? do = object distance from lens di = image distance from lens do = 2m f = 1m di = ? f = distance from lens to focal point (focal length) 1/di + ½ = 1/1 1/di = 1 – ½ 1 1 1 1/di = .5 Lens Equation + = di = 2 m do d i f 1. An object placed 30 cm in front of a converging lens forms an image 15 cm behind the lens. What is the focal length of the lens? 2. A converging lens with a focal length of 20 cm is used to produce an image on a screen that is 2.0 m from the lens. What is the object distance? 3. For a biconvex lens, what is the minimum distance between an object and its image if the image is a. real? b. virtual? 4. If a book is held 30 cm from an eyeglass lens with a focal length of a. -45 cm, where is the image of the print formed? b. +57 cm is used, where is the image formed? 5. The geometry of a compound microscope, which consists of two converging lenses is shown below. The objective lens and the eyepiece lens have focal lengths of 2.8 mm and 3.3 cm, respectively. If an object is located 3.0 mm from the objective lens, where is the final image located and what type of image is it? (Draw a picture) 6. Two converging lenses L1 and L2 have focal lengths of 30 cm and 20 cm respectively. The lenses are placed 60 cm apart along the same axis, and an object is placed 50 cm from L1 on the side opposite L2. Where is the image formed relative to L2, and what are its characteristics? (Draw a picture) 11
12. 12. More Ray Diagrams Directions: 1. Draw Ray diagram 2. Compare to the lens equation 12
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15. 15. Six Cases of converging lenses In the convex lens lab, you looked at the images formed by a converging lens when the object was in different locations. Today you will draw ray diagrams for the same six object positions on the back of this page and analyze the images in the exact same way. After you have completed the six lens diagrams, complete the summary data table below and compare it to your results from the convex lens lab. Then look for trends in your results. Case 1: Object at Infinity Case 4: Object between 2F & F Case 2: Object beyond 2F Case 5: Object at F Case 3: Object at 2F Case 6: Object closer than F Trends: 1. As the object gets closer to the focal point, the image location gets _____________. 2. As the objects gets closer to the focal point, the image size gets _______________. 3. The only virtual image is formed when the object is located ___________________. The size of the virtual image is _______________. The occurs in case _______ which is called the magnifying glass case. 4. Real images are always ______________ compared to the object. 5. Real images are formed by converging lenses when the object is located ______________. 6. It is impossible to get an image when the object is located _______________. 7. The object and image are the same size and same distance from the lens when the object is located __________________. case Object Image Image type- Image size Image location location real or virtual orientation- RSU or USD 1 2 3 4 5 6 15
16. 16. Case 1: (the object is way off the page to the left <-----------) 2F F F 2F Case 2: Case 3 Case 4 Case 5 Case 6 16