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ISU ENVI 360 - Optics MiniLab Fall 2015

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Optics/Lenses Mini-Lab____________________________________Purpose- Investigate the properties of thin lenses.- Investigate the design of refracting telescopes- Make a simple two-lens telescope.Background InformationOptical instruments are used in many ways. Made with lenses, mirrors or both, such instrumentsare used in everyday life in science, industry, medicine and in practically every facet of ourtechnological society. Each instrument produces an image that allows one to observe an object.The simplest optical instrument, a magnifying glass, is a single lens that can be used to makeobjects appear larger. Eyeglasses are also single lenses; they are custom-made to correct eachperson’s visual deficiencies. Microscopes enlarge tiny objects so that they can be seen in detail.Telescopes enlarge far-away objects. Although optical instruments may be quite complex, theyare all based upon the effects of reflection and refraction. The simple cases of reflection from aspherical mirror and refraction through a thin lens with spherical surfaces are the easiest to study.Though simple, they are the basis of many complicated instruments; therefore, much can begained by understanding simple mirrors and lenses.Rays of light leave a point source traveling in all directions. When some of those rays arereflected from a mirror or pass through a lens and converge to a point image at a distance calledthe focal length (f), we say that the image is real. When the rays leaving a point diverge afterreflection or refraction but appear to come from an image point, the image is virtual (see Figure1). If the image is produced by a single mirror or lens, a real image will be inverted (upsidedown compared to the object) and a virtual image will be upright when compared to the object.For our lenses, f is pretty small so that the distance from the lens to projected image for eitherlens should be pretty close to the focal length for that lens for each part of the investigation.1Object Image f f i o Figure 1. Real object produced by a convex lens. The letter o represents the object distance from the lens, i is the image distance from the lens, and f is the focal length of the lens. The magnification of the image formed by the lens can be found by either by dividing the imageheight (hi) by the object height (ho) or by dividing the negative of the image distance by theobject distance. The “actual” magnification is defined by the actual image that you seem=hiho. (1)One “should” get the same result by measuring distances between the image and the lense, andthe distance between the lens and the objectm=−dido. (2)When the image is inverted (upside down) as in this illustration, the height of the image is a negative value.2For a real image, i is positive and the magnification is negative, which represents the fact that theimage is inverted with respect to the object. For a virtual image, the magnification is positivebecause the image is upright when compared to the object. The sign of the magnification willwork out automatically when you use the image and object positions. However, when you usethe image heights both values are positive so you must determine the sign of themagnification by determining if the image is upright or inverted. When the absolute value ofthe magnification is m  1 the image is smaller than the object that produced it. If the absolutevalue of the magnification is m  1 the image is larger than the object.The first lens (the one located opposite the eyepiece) in a telescope is called the Objective, andits job is to make this real image inside the tube of the telescope. To complete the telescope, asecond lens is used as a magnifying glass, called the Eyepiece, to look at the image made by theObjective. The magnifying power M (not the same as m in equation 1 above) of the telescope isapproximately the ratio of the focal lengths of these two lenses:M =FobjFeye(3)3Equipment ListLight source2 converging lenses (100 mm and 200 mm)RulerA Sheet of White PaperMeter StickI. Procedure and Analysis (Record answers on Lens Data Sheet)1. Estimate the distance to the object (Arrow) at the front of the room.2. One person from each group come to the front of the room and record the height of theobject/Arrow.3. Adjust the distance between the 100 mm lens and the sheet of white paper until you form aclear image of the Arrow on the sheet of white paper. The Arrow Object at the front of theroom is “far away,” the distance between the lens and the paper should be a good estimate ofthe focal length. Record this distance as the focal length f1, and also note whether the imageis Upright or Inverted (upside down)4. Note whether the image of the Arrow Object is upright or inverted.5. Cover half of the lens with a piece of paper. Record in your table what happens to the image.Is this what you expected?6. Measure the height of the image of the Arrow Object that is on your sheet of paper.7. Calculate the magnification of the image using these distances and equation 1. Show thiscalculation, including units.9. Using the other lens as a magnifying glass, look at the image of the Arrow Object on yoursheet of white paper. Compare how large the Arrow Object looks when you look directly atit from your seat, with the magnified image on your view screen. Which seems to be larger?Record your observations.10. REPEAT STEPS 5 – 9 using the 200 mm focal length lens.II. Complete the Simple Telescope Data Sheet (Record answers on Data Sheet).4Lenses Data Sheet Names: _______________________ #______________________________ #______Object MeasurementsDistance to Arrow Object: ________Height of Arrow Object: ________The 100 mm LensEstimate of Focal Length of the Lens by measuring the distance from the lens to the image on thesheet of white paper f1 = ________ Is the Image Upright or Inverted: ________Cover half of the lens. What is the effect on the image? Was this expected?Describe the size of the Magnified Image compared to the size of the Arrow Object and calculatethe magnification using Eq. 1:Magnification of Arrow Object (equation 2, using the object distances, show your work).Compare this with the value and your description in the previous step.The 200 mm LensEstimate of Focal Length of the Lens by measuring the distance from the lens to the image on thesheet of white paper f2 = ________ Is the Image Upright or Inverted:


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ISU ENVI 360 - Optics MiniLab Fall 2015

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