A 2.0-cm-diameter spider is 3.0 m from a wall. Part A Determine the focal length

Question

A 2.0-cm-diameter spider is 3.0 m from a wall.

Part A

Determine the focal length f and position s (measured from the wall) of a lens that will make a double-size image of the spider on the wall.

Express your answers using two significant figures separated by a comma.

A 2.0-cm-diameter spider is 3.0 m from a wall.

Part A

Determine the focal length f and position s (measured from the wall) of a lens that will make a double-size image of the spider on the wall.

Express your answers using two significant figures separated by a comma.

Explanation / Answer

Magnified and image is required, so convex lens is being used

Consider real image on the wall
so the distance of the screen(wall) from lens = v
distance of the object from the lens = -(3 - v)
now 1/f = 1/u + 1/v = -1/(3 -v) + 1/v
but v/(3-v) = 2
or v = 6 - 2v or v = 2m

so 1/f = 1/1 + 1/2 = 3/2
f = 2/3 = 0.66m
s' = 2m

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