When a converging lens is used in a camera (as in the first drawing), the film m

Question

When a converging lens is used in a camera (as in the first drawing), the film must be placed at a distance of 0.220 m from the lens to record an image of an object that is 4.20 m from the lens. The same lens is then used in a projector (as in the second drawing), with the screen 0.410 m from the lens. How far from the projector lens should the film be placed?

Explanation / Answer

keep in mind these equations 1) 1/object + 1/image= 1/focal length 2) magnification = image height/object height= distance of image/distance of object. use eq1), with image distance=0.210m and object distance = 4.00m this gives you the focal length: 1/4+1/0.21= 1/f => f=0.2 m now, use this value of f (since you are using the same lens) to find the object distance knowing the image distance is now 5.6m 1/o + 1/5.6=1/0.2 1/o=1/0.2-1/5.56 => o=0.21m

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