1. The problem statement, all variables and given/known data 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?