People doing detailed work occasionally have a lens on a swing-arm that they can

Question

People doing detailed work occasionally have a lens on a swing-arm that they can bring in between an object and their eyes. Suppose we are looking at a 2 cm tall stamp on the table, Presently located 30 cm from our eye. We slide a lens in place such that the lens is 10 cm from the stamp. The image of the stamp now appears to be 50 cm away from our eye, and is still upright. (You'll definitely want a figure here to aid in getting all the parameters you need.) (a) What must be the focal length of this lens? _____ cm

Explanation / Answer

given distance between lens and object =u=10 cm

distance between eye and lens=30-10=20 cm

distance between lens and image=v=50-20=30 cm

using

1/v + 1/u=1/f

1/(-30) +1/10=1/f

we got f=15 cm

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