Front Focal

A stamp collector uses a converging lens with focal length 21 cm to view a stamp 11 cm in front of the lens.?

Calculate the the distance between the lens and the image.

Use 1/f = 1/s + 1/s’ to find s’ (the image distance)

So 1/s’ = 1/f – 1/s or s’ = f*s/(s – f) = 21*11/(11 – 21) = -23.1cm

so the distance is 23.1 cm in front of the lens (that’s what the negative sign means –also virtual image)

m = -s’/s = -(-23.1/11) = 2.1

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