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1) In measuring focal length Measuring the Focal Length of a Positive Lens Using bisplacement Method. If the distance between the object and the screen is at least four times of the focal length and when w move the lens, we can get a clear image twice at different points as shown in Figure 2-3. Figure 2-3 Schematic of thin lens itmging Correspondingly, we get the two following equations: =i+. (2-[} S = + . (2-2} Usingthccondilions: =sl +3; =3: +353, =3, +d,3: =.i:'2 +d,wccangctthc formula: :_ t #040\" (2-3) Explain why should the distance between object and screen be at least four times of the focal length