What is the Unknown Distance in a Lens Equation Problem?

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The discussion revolves around solving a lens equation problem where the focal length is 5 cm and the object distance is 100 cm. The key equation used is (1/f) = (1/p) + (1/q). A participant expresses confusion about the original object distance, referred to as P, which is needed to solve for q. Another participant clarifies that since the light rays are parallel from a distant object, the original P is considered to be infinity, simplifying the equation. This understanding allows for the calculation of the unknown distance in the lens equation problem.
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Homework Statement



http://img368.imageshack.us/img368/9489/physicszv8.jpg


Homework Equations



(1/f) = (1/p) + (1/q)

The Attempt at a Solution



I tried solving this but I don't know what the original P was so I can't solve for q. How can I solve this?
 
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The focal length is 5cm and the object distance is 100cm. You only have one unknown variable, so it should be easy to solve.
 
petern said:
I tried solving this but I don't know what the original P was

Hi peter!

In diagram (a), the light rays were parallel, "from a very distant object".

So the original p was infinity. :smile:

And 1/infinity = … ? :smile:
 
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