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UW-Madison PHYSICS 208 - Lenses and the eye

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Name _________________________________ Section ________ Physics 208 Spring 2008 Lab 2: Lenses and the eye Your TA will use this sheet to score your lab. It is to be turned in at the end of lab. You must use complete sentences and clearly explain your reasoning to receive full credit. What are we doing this time? You will complete three related investigations. PART A: Use a single lens on the optical track to understand focus and the lens equation, and the difference between real and virtual images. PART B: Build a microscope and a telescope on the optical track to understand how the image formed by the objective lens acts as an object for the eyepiece lens. PART C: Investigate near-sightedness and far-sightedness using an eye model filled with water. Why are we doing this? Refraction of light is an unusual phenomenon, and the way lenses use this effect to bend and reform light rays to make an image has myriads of applications. Understanding this will help you get the most out of research instruments such as microscopes. What should I be thinking about before I start this lab? You should be thinking about waves, and how the phase difference between waves causes constructive and destructive interference. Any safety issues? If you aim your telescope across the street at the windows in the chemistry building, just keep an open mind and remember that chemists are different.2 A. Lenses Aim Firefox at the course web site laboratories page and click on the “Virtual Optics Bench” for Lab 1. Click the top button for ‘Converging Lens’, and scroll the page to look at the lens system. A1. Move the lens around to see the locations at which an image forms on the screen. Then move the screen closer and try again. Are there always the same numbers of lens positions at a sharp image forms on the screen? Explain what you found. Now you will use the Pasco track and light source, three color-coded converging lenses, and a white screen to view images. Position the light source accurately at 0 cm on the track, and the white screen at 30 cm. Put one of the red lenses on the track between the screen and the light source. A2. Find all the positions of one of the red lenses that give a sharp image on the screen (there is more than one position, but there may not be four) Lens position (cm) Image distance (cm) Object distance (cm) Magnification3 A3. From the data of A2, use the lens equation to calculate the red lens focal length. The two red lenses have nominally the same focal length, but may differ by several mm. A4. Now move the screen to 100 cm, and write down the positions of the green lens that gives a sharp image on the screen. A5. Calculate the focal length of the green lens.4 Magnifying glass (simple magnifier) Go back to the “Virtual Optics Bench” from the course web site and click the button for “Magnifier w/Eye Lens”. Move the magnifying glass and eye around, and look at the rays. Use the simulation to answer the following question. A6. The eye can focus only on objects more than 25 cm away. Put the eye 6 cm behind the magnifying glass. What is the closest to the object that the magnifying glass can be held? A7. Now use a Pasco red lens as a magnifying glass. That is, hold the lens in your hand close to the words on this page so that they appear larger when you look through the magnifying glass. Move the magnifying glass and your eye until the words are as big as possible, but still in focus. The image may be distorted due to imperfections in the lens. To answer these questions, it may help to refer back to the simulation. i) Are the words on the page closer to or farther from the lens than the focal length? ii) Is the image formed by the magnifying glass upright, or inverted? iii) Is the image formed by the magnifying glass real or virtual? Explain. . iv) Now try the green lens, then answer this question: which lens can magnify the most when used as a magnifying glass, long focal length or short focal length? .5 B. Microscopes and Telescopes This section involves two-lens systems. These will make a lot more sense if you remember these things: 1. The first lens (objective) forms a real image. 2. The image formed by the objective is used as the object for the (eyepiece) lens 3. A real image will appear on a screen placed at the image location. A virtual image will not, but your eye can focus the rays from a virtual image on your retina. Compound Microscope Go back to the “Virtual Optics Bench” from the course web site and click the “Microscope” button. Move the objective with respect to the object and watch the image locations change. B1. How does the position of the image formed by the objective move when you change the objective position? B2. Is the image formed by the objective real or virtual? . B3. How does the position of the image formed by the eyepiece move when you change the objective position? B4. Does the eyepiece form a real or virtual image? B5. Why does changing the distance between the objective and the object (moving the stage in a microscope) focus the microscope?6 Now use the two red lenses to make a compound microscope, an optical instrument that makes a nearby sample appear larger. The objective (lens closest to the sample) is used to make a larger, real image of the sample. The eyepiece (lens closest to your eye) is used as a magnifying glass to look more closely at the image from the objective. B6. Use one of the red lenses as the microscope objective to form an image of the light source on your white screen somewhere on the track. Measure the positions of the objective and image and fill in the following table: Distance of objective from light source Distance of image from objective B7. Approximately at what location on the track should you position the other red lens (as the eyepiece) so that you can look through the eyepiece and see a magnified image of the light source? B8. Complete your microscope by putting the eyepiece at the appropriate location, and experimentally estimate the magnification power of your microscope using any method you can think of.7 B9. Draw your microscope here. Remember that the objective lens will form a real image that you can see on a screen. The eyepiece is used as a magnifying glass to


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UW-Madison PHYSICS 208 - Lenses and the eye

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