4.6 Focal length of lens combinations Task

Science - Physics - Optics - 4 Lenses (P1065800)4.6 Focal length of lens combinationsExperiment by: linear motion wiht timerPrinted: Jan 10, 2011 12:03:07 PMinterTESS (Version 10.03 B132, Export 1678)TaskTaskWhat advantage do lens combinations offer?Determine the focal length of planoconvex lenses, biconcave lenses and various lens combinations.Use the space below for your own notes.Logged in as a teacher you will find a button below for additional information.- 1 -

MaterialMaterialMaterial from "TESS-Optics OE 1" (Order No. 13276.88)Position No.1MaterialBlock, semicircular, r = 30 mmOrder No.09810.01Quantity12, 3Block, planoconvex lens, f = +100 mm09810.0424Block, planoconcave lens, f = -100 mm09810.0515, 6Light box, halogen 12 V / 20 W09801.001Additional MaterialPower Supply, 0...12 V DC / 6 V, 12 V AC13505.931White paper (DIN A4)1Ruler (approx. 30 cm)1Material required for the experiment- 2 -

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SetupSetupAttentionTake care that the lenses lie with their flat sides precisely on the perpendicular of the crossedlines and that their position does not change during the experiment.Draw crossed lines at right angle to each other in the middle of your sheet of paper. Theintersectionof the lines is point M.Make two marks, one on either side of M, at a distance of 3 cm away from M.Fig. 1Lay the planoconvex lens (rough side downwards) with the flat side precisely on the perpendicularline within the two marks.Insert the three-slit aperture in the light box on the lens side and position this about 10 cm fromthe plane edge of the block.Fig. 2- 4 -

ActionActionConnect the light box to the power supply (12 V AC).Fig. 3Move the light box and, if necessary, the lens carefully until the middle light beam travels preciselyalong the optical axis and on passing the lens is not refracted.Observe the course of the parallel light beams on passing through the lens and note yourobservations in the table on the results page.Fig. 4Mark the position of the focal point on the optical axis and label it F 1.- 5 -

Fig. 5Step by step, change the arrangement as pictured.Describe the observed light path in the table on the results page and mark the intersection of thelight beams with the optical axis for:- A symmetrical biconvex lens; the intersection is F 2.Fig. 6- A non-symmetrical biconvex lens; the intersection is F 3.Fig. 7- Lens combination 1; the intersection point is F 4.- 6 -

Fig. 9- Lens combination 2; the intersection point is F 5.Fig. 10Switch off the power supply and remove the light box and the lenses from the paper.- 7 -

ResultsResultsLens in the light pathObservation on the path of the light beamsPlanoconvex lensSymmetrical biconvex lensNon-symmetrical biconvex lensLens combination 1Lens combination 2- 8 -

EvaluationEvaluationQuestion 1Determine the distance f of the point M from the individual focal points F 1, F 2, F 3, F 4and F 5(focallength) in metres (m) write the values in the corresponding line of the table below.Lens in light pathPlanoconvex lensSymmetrical biconvex lensNon-symmetrical biconvex lensLens combination 1Lens combination 2f in mnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnD in dptnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnQuestion 2In the optical industry, the value of the refractive power D for lenses combinations is given indioptres (1 dpt = 1/m). The refractive power D is given by the inverse of the focal length f:D = 1/ f.Calculate the refractive power of the lenses in dioptres and write these values in the correspondingcolumns of the table above.Question 3Is the refractive power of the combination of two planoconvex lenses larger or smaller than of theindividual lenses?- 9 -

Question 4Does the refractive power of the lens combination depend on the order in which the lenses arearranged in the light path?Question 5What advantage do lens combinations have?Supplementary problemSome of the values obtained for the focal lengths of lens combinations with the above procedurediffer substantially from the true values.What causes could there be for this?- 10 -