Experiment 51Designing ExperimentforGeometricalOpticsExperiment51DesigningExperimentforGeometricalOpticsTelescope and microscope are the most common vision-aid optical instruments,and thefrequentlyused ones in physical experiment arenumerical reading microscope,survey telescope, autocollimating telescope, etc. In this experiment, you are expectedto build thetelescopic system and microscopic system using the individual opticalelements provided by the laboratory, and to measure the focal length of the lens.Experimental objectives1.Understand the imaging rules of the thin lens2.Learn the basic structures and the working principles of the telescope andmicroscope3.Masterthemethods of measuring thefocal length of a lensExperimental instrumentsConvex lens with a focal length not exceeding 25 cm, concave lens, illuminantobject screen,object screen,reticle,white screen,optical bench with scaleExperimentalprinciples1.Measuring thefocal lengthoftheconvex lensThe beam path of a point lightsource focusing through a lens isLoPshown in Figure 51-1. D (D >4f)Fo"represents the distance of the pointsource(illuminantobjectscreen)fromthe white screen (image screen)Changing the position of the convexlens between the point source and thewhite screen, two real images withFigure 51-1 Beam path for pointdifferent size will appear on the screen,sourcefocusing throughthethin lenswhich is called object-image conjugate.y represents point light source,P is thewhite screen, and Lo represents the thin convex lens. Let d is the distance between thetwo images, one can obtain thefocal length equations of the thin lenses according tothe basic equations forthin lenses.Read the positions of the obiect y.thin convex lensLoand screenPontheruler of theoptical bench,andtrytocalculatethefocal lengthof the lens Lo
Experiment 51 Designing Experiment for Geometrical Optics 1 Experiment 51 Designing Experiment for Geometrical Optics Telescope and microscope are the most common vision-aid optical instruments, and the frequently used ones in physical experiment are numerical reading microscope, survey telescope, autocollimating telescope, etc. In this experiment, you are expected to build the telescopic system and microscopic system using the individual optical elements provided by the laboratory, and to measure the focal length of the lens. Experimental objectives 1. Understand the imaging rules of the thin lens 2. Learn the basic structures and the working principles of the telescope and microscope 3. Master the methods of measuring the focal length of a lens Experimental instruments Convex lens with a focal length not exceeding 25 cm, concave lens, illuminant object screen, object screen, reticle, white screen, optical bench with scale Experimental principles 1. Measuring the focal length of the convex lens The beam path of a point light source focusing through a lens is shown in Figure 51-1. D（D >4f） represents the distance of the point source (illuminant object screen) from the white screen (image screen). Changing the position of the convex lens between the point source and the white screen, two real images with different size will appear on the screen, which is called object-image conjugate. y represents point light source, P is the white screen，and Lo represents the thin convex lens. Let d is the distance between the two images, one can obtain the focal length equations of the thin lenses according to the basic equations for thin lenses. Read the positions of the object y, thin convex lens Lo and screen P on the ruler of the optical bench, and try to calculate the focal length of the lens Lo. Figure 51-1 Beam path for point source focusing through the thin lens Fo′ Lo′ P y y’ Lo″ y″ Fo″ D d
Experiment51DesigningExperimentforGeometrical Optics2.MicroscopeThe optical device that is usedtoobservethetinyparticleisLomicroscope, and the light path of amicroscope is shown in FigureV51-2.Neartheobjectundertest isobjective, and the focal length ofwhich is short. Near your eyes iseyepiece, and its focal length isslightly longer. In order to find theaccurate position of the object, putFigure 51-2 Light path of a microscopethe reticle P between the objectiveLo and the eyepiece Le, adjust thespace of eyepiece Le and reticle P, and make the corsshair on the reticle P to be asclearaspossibleThe object screen y is placed away from the focal point Fo of the objective lens Lo.By adjusting the distance between the object screen y and the objective lens Lo, theobject screen y is formed into an enlarged and inverted image y' on reticle PObserving the image y'through the eyepiece Le, a magnified and inverted virtual imagewill be found overlapped with the crosshair on reticle P, so that the tiny object y ismagnified intoy"BychangingthedistancebetweenPandLo,differentmagnificationof the microscope can be obtained.3.TelescopeCommon telescopes can beLodivided into Galileo telescope,Kepler telescope and Newtoniantelescope.The optical path ofKepler telescope is shown inFigure 51-3. The focal length ofthe objective lens is longer thanthat of the eyepiece lens. TheFigure 51-3Light pathofKepler telescopelight from the object in aninfinite distance (parallel light) passes through the objective Lo to form a real image y'which is atthefocal planeoftheobjectivelens Land withinthefocal lengthof theeyepiece Le. Change the position of objective Lo, so that the real image y' coincideswith reticle P (themost clear position observed by eyepiece Le).The process ofwatching image y" through the eyepiece Le of the telescope is similar to that by amicroscope. That is, to observe objects through a telescope is equivalent to pullingdistant objects close to the observer, essentially playing the role of anglemagnification.2
Experiment 51 Designing Experiment for Geometrical Optics 2 2．Microscope The optical device that is used to observe the tiny particle is microscope, and the light path of a microscope is shown in Figure 51-2. Near the object under test is objective, and the focal length of which is short. Near your eyes is eyepiece, and its focal length is slightly longer. In order to find the accurate position of the object, put the reticle P between the objective Lo and the eyepiece Le, adjust the space of eyepiece Le and reticle P, and make the corsshair on the reticle P to be as clear as possible. The object screen y is placed away from the focal point Fo of the objective lens Lo. By adjusting the distance between the object screen y and the objective lens Lo, the object screen y is formed into an enlarged and inverted image y' on reticle P. Observing the image y' through the eyepiece Le, a magnified and inverted virtual image will be found overlapped with the crosshair on reticle P, so that the tiny object y is magnified into y″. By changing the distance between P and Lo, different magnification of the microscope can be obtained. 3．Telescope Common telescopes can be divided into Galileo telescope, Kepler telescope and Newtonian telescope. The optical path of Kepler telescope is shown in Figure 51-3. The focal length of the objective lens is longer than that of the eyepiece lens. The light from the object in an infinite distance (parallel light) passes through the objective Lo to form a real image y', which is at the focal plane of the objective lens Lo and within the focal length of the eyepiece Le. Change the position of objective Lo, so that the real image y' coincides with reticle P (the most clear position observed by eyepiece Le). The process of watching image y'' through the eyepiece Le of the telescope is similar to that by a microscope. That is, to observe objects through a telescope is equivalent to pulling distant objects close to the observer, essentially playing the role of angle magnification. Fig.51-2 Light path of a m Fo Fe Lo P Le u υ l y y′ y″ Figure 51-2 Light path of a microscope Figure 51-3 Light path of Kepler telescope Fe Fo Lo P Le y″ y′
Experiment 51Designing ExperimentforGeometricalOpticsExperimental contentandprocedure1.Measure the focal length of each convex lens by object-image conjugate method.Draw the optical path diagram, write the relevant formula, record the experimentaldata and calculatethe focal length.(hint:If the focal length of the lens is found tobe too short, the space between the illuminant object screen y, the thin convex lensLo and the white screen P can be appropriately shortened to ensure the imageclarity)2.Accordingtoyourinitial results,selecttwo convexlensestobuild atelescopewitha reticle. It also requires focusing on infinity and being able to zoom in as much aspossible.3.Use this self-assembled telescope focusing on infinity to measure the focal lengthof another convex lens (choose only one of the left lenses). Draw the light pathdiagram and the data table. Give the focal length or the number of the selectedobjective and eyepiece, and record the distance between the object screen and themeasured convex lens,objective and crosshair,crosshair and eyepiece of thetelescope system. Compare the experimental results with that by the previousobject-image conjugate method.4. Use this self-assembled telescope focusing on an infinite distance to measure thefocal length of a concave lens. The experimental principle is that the object screenforms parallel light through a convex lens and a concave lens, and then thetelescope system receives parallel light. On the basis of the experiment above (step3), move the object screen away from the objective. (The smaller the focal lengthof the selected convex lens, the further the object screen is away from the lens).Insert the measured concave lens between the convex lens and the objective of thetelescope.Move the concave lens back and forth until the image on the reticle isclearly seen through the telescope and the parallax eliminated. Draw the opticalpath diagram, give the formula for calculating thefocal length of the concave lens,mark therelevantparameters,drawthedatatable,and calculatethefocal lengthoftheconcavelens.5.Choose two convex lenses spaced about 15 cm apart to set up a microscope with areticle and the largest magnification. How do you choose the convex lenses? Drawthe optical path diagram and the data table, mark the focal length (or lens number)of the selected objective and eyepiece, and give the distance between the objectscreen and objective, objective and crosshair, crosshair and eyepiece.Questions:1) How to judge the convexity or concavity of a lens (do not touch it by hand)? Writedown the judgment method and results.2) When using the object-image conjugate method to measure the focal length of a3
Experiment 51 Designing Experiment for Geometrical Optics 3 Experimental content and procedure 1. Measure the focal length of each convex lens by object-image conjugate method. Draw the optical path diagram, write the relevant formula, record the experimental data and calculate the focal length. (hint: If the focal length of the lens is found to be too short, the space between the illuminant object screen y, the thin convex lens Lo and the white screen P can be appropriately shortened to ensure the image clarity) 2. According to your initial results, select two convex lenses to build a telescope with a reticle. It also requires focusing on infinity and being able to zoom in as much as possible. 3. Use this self-assembled telescope focusing on infinity to measure the focal length of another convex lens (choose only one of the left lenses). Draw the light path diagram and the data table. Give the focal length or the number of the selected objective and eyepiece, and record the distance between the object screen and the measured convex lens, objective and crosshair, crosshair and eyepiece of the telescope system. Compare the experimental results with that by the previous object-image conjugate method. 4. Use this self-assembled telescope focusing on an infinite distance to measure the focal length of a concave lens. The experimental principle is that the object screen forms parallel light through a convex lens and a concave lens, and then the telescope system receives parallel light. On the basis of the experiment above (step 3), move the object screen away from the objective. (The smaller the focal length of the selected convex lens, the further the object screen is away from the lens). Insert the measured concave lens between the convex lens and the objective of the telescope. Move the concave lens back and forth until the image on the reticle is clearly seen through the telescope and the parallax eliminated. Draw the optical path diagram, give the formula for calculating the focal length of the concave lens, mark the relevant parameters, draw the data table, and calculate the focal length of the concave lens. 5. Choose two convex lenses spaced about 15 cm apart to set up a microscope with a reticle and the largest magnification. How do you choose the convex lenses? Draw the optical path diagram and the data table, mark the focal length (or lens number) of the selected objective and eyepiece, and give the distance between the object screen and objective, objective and crosshair, crosshair and eyepiece. Questions： 1) How to judge the convexity or concavity of a lens (do not touch it by hand)? Write down the judgment method and results. 2) When using the object-image conjugate method to measure the focal length of a
Experiment 51Designing Experimentfor Geometrical Opticsconvex lens, what is the requirement for the distance between the illuminant objectscreen and the white screen?How many images can you find on the white screenby changing the position of the convex lens?3) How do you adjust the objective lens when observing objects with limited distanceby this self-assembled telescope focusing on infinity?What are the principles for selecting objective and eyepiece lenses to maximize4the magnification of the distant objects? What happens if you switch the positionsof the objective and eyepiece lenses of this self-assembled telescope?5)What are the principles for selecting the objective and eyepiece lenses of themicroscope? What happens when you switch the positions of the objective andeyepiecelenses?The first 5 questions are required. If the answer is not certain, youcan try to do it in the experiment class and then give the answers.The remaining 4 questions below are for thought and exercise.6) In Figure 51-3, once the eyepiece is selected, what factors would determine thedistance between Le and image y'?7) When viewing the object screen with convex lens #1, what is the reason forfinding that the clear position of the vertical red scale on the object screen is notconsistentwiththatofthehorizontal black scale?Howtoverifyyour answers?8)How many types of telescopes are there? What are the strengths and weaknessesof them? What is the role of concave lens in modern telescopes?9) What are the similarities of the telescopes and microscopes, and what are the maindifferences between them? Compare their differences in aspects of utilization,structure, magnification, and focusing methods
Experiment 51 Designing Experiment for Geometrical Optics 4 convex lens, what is the requirement for the distance between the illuminant object screen and the white screen? How many images can you find on the white screen by changing the position of the convex lens? 3) How do you adjust the objective lens when observing objects with limited distance by this self-assembled telescope focusing on infinity? 4) What are the principles for selecting objective and eyepiece lenses to maximize the magnification of the distant objects? What happens if you switch the positions of the objective and eyepiece lenses of this self-assembled telescope? 5) What are the principles for selecting the objective and eyepiece lenses of the microscope? What happens when you switch the positions of the objective and eyepiece lenses? The first 5 questions are required. If the answer is not certain, you can try to do it in the experiment class and then give the answers. The remaining 4 questions below are for thought and exercise. 6) In Figure 51-3, once the eyepiece is selected, what factors would determine the distance between Le and image y'? 7) When viewing the object screen with convex lens #1, what is the reason for finding that the clear position of the vertical red scale on the object screen is not consistent with that of the horizontal black scale? How to verify your answers? 8) How many types of telescopes are there? What are the strengths and weaknesses of them? What is the role of concave lens in modern telescopes? 9) What are the similarities of the telescopes and microscopes, and what are the main differences between them? Compare their differences in aspects of utilization, structure, magnification, and focusing methods