How Do You Calculate Object Distance in Lens Equations?

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To calculate the object distance in lens equations, the formula used is 1/f = 1/di + 1/do. In the given problem, the focal length (f) is 56.0 mm and the image distance (di) is 68.0 mm behind the lens. The correct approach is to use di as a positive value, leading to the equation 1/56 = 1/68 + 1/do. The initial confusion arose from incorrectly adding the distances instead of properly applying the lens formula. Ultimately, the correct calculation reveals that the object distance (do) is determined by accurately solving the equation without mixing up the values.
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The homework problem is "A sharp image is located 68.0 mm behind a 56.0 mm focal length converging lens. Calculate the object distance in millimeters"



1/f = 1/di + 1/do



I tried solving based on having focal (f) and distance of image (di) so 1/56 mm = 1/(68 mm + 56 mm) + 1/do
I solved for do coming up with 102.04 mm and the computer doesn't accept
 
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Why is di=68+56?
 
I figured because the focal point was 56 and it says 68 behind it so I thought it would be f distance + the 68
 
It states it is behind a 56mm focal length lens. Not 68mm behind the focal length of a 56mm focal length lens.
 
I also did it as 1/56 = 1/68 + 1/do and the computer didnt accept that on as well
 
ok I figured this one out. I had my numbers flip flopped my numbers when subtracting
 

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