Thin lens equation and problem solving (video) Khan Academy. The focal length, when you've got a thin lens, there's a focal point on each side of the lens. The focal length is the distance from the center of the lens to one of these focal points. Which one, it's doesn't actually matter, because if you want to know whether the focal length is positive or negative, all you have to look at is what type of lens you have.
The thin lens equation shows the relationship between the image distance ( di i ), object distance ( do o ), and the focal length ( f) of the lens. It is expressed as 1 do + 1 d =.
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