A 5.29-mm-high firefly sits on the axis of, and 13.1 cm in front of, the thin le

Question

A 5.29-mm-high firefly sits on the axis of, and 13.1 cm in front of, the thin lens A, whose focal length is 6.79 cm. Behind lens A there is another thin lens, lens B, with focal length 20.3 cm. The two lenses share a common axis and are 63.3 cm apart. Is the image of the firefly that lens B forms real or virtual? How far from lens B is this image located (expressed as a positive number)? What is the height of this image (as a positive number)? Is this image upright or inverted with respect to the firefly?

Explanation / Answer

lens A

s1 = 13.1 c

1/s1 + 1/s1' = 1/f1

(1/13.1)+(1/s1')=(1/6.79)

s1' = 14.09 cm

m1 = -s1'/s1 = -1.075

for lens B

s2 = 63.3 - 14.09 = 49.21 cm

1/s2 + 1/s2' = 1/f2

(1/49.21)+(1/s2') = (1/20.3)

s2' = 34.55 cm

m2 = -s2'/s1' = -0.702

M = m1*m2 = 0.75465

real

s2' = 34.55 cm

hi = M*ho = 3.992 mm

upright

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