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Homework Statement
An opaque card is pierced by two small holes 4 mm apart and strongly illuminated from one side. A lens on the other side of the card focuses images of the holes, 16 mm apart, on a screen 125 cm from the card. Find: (a) the position of the lens, (bc) the focal length. Why are measurements of the object and image sizes in this experiment arrangement unlikely to yield a reliable value for the focal length of the lens?
Answers: (a) 25 cm from card, (b) 20 cm.
2. The attempt at a solution
(b) 1 / u + 1 / v = 1 / f
1 / 25 + 1 / (125-100) = 1 / f
f = 20 cm
Besides that I don't quite understand how to find the 25 cm from the card. I also don't quite understand what do the 16 mm represent.
This is how see it:
We have two holes which are 4 mm apart. We need to find the distance to the lens (a). 16 mm (wrongly on the image cm) is the distance between the projected light.
Having all this I don't know what to proceed with.
For the "Why are measurements of the object and image sizes in this experiment arrangement unlikely to yield a reliable value for the focal length of the lens?" maybe this is because of the two holes and not one?
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