A 2.0-cm-diameter spider is 2.2 m from a wall.
Determine the focal length f and position s′ (measured from the wall) of a lens that will make a triple-size image of the spider on the wall.
1 Answer
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if s’ is measured from the wall to the lens in metres:
(1/f) = (1/dₒ) + (1/dᵢ)
=> (1/f) = [1/(2.2 – s’)] + (1/s’) ————————> (i)
M = magnification = – dᵢ/dₒ = hᵢ/hₒ = -3 (-tive for an inverted image)
=> dᵢ = +3dₒ
from equation (i) => s’ = 3(2.2 – s’)
=> 4s’ = 6.6
or s’ = 1.65 m
and (1/f) = [1/(2.2 – 1.65′)] + (1/1.65) = 2.424
=> f = +0.4125 m (+tive indicates that it is a converging lens)
dₒ = 0.55 m, dᵢ = 1.65 m
hope this helps
