Translations:Adventist Youth Honors Answer Book/Health and Science/Optics/41/en
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Once focused, we can measure the distance between the negative lens and the image on the screen and call it [math]\displaystyle{ v_2 }[/math]. The only other information we need for calculating the focal length of the negative lens is the distance between the negative lens and its object. But where is its object? It lies between the negative lens and the screen at the positive lens's focal point (which you have presumably already measured).
