A 5.87-mm-high firefly sits on the axis of, and 10.5 cm in front of, the thin le

Question

A 5.87-mm-high firefly sits on the axis of, and 10.5 cm in front of, the thin lens A, whose focal length is 6.65 cm. Behind lens A there is another thin lens, lens B, with focal length 27.3 cm. The two lenses share a common axis and are 61.9 cm apart. Is the image of the firefly that lens B forms real or virtual? O Real O Virtual How far from lens B is this image located (expressed as a positive number)? Number cm What is the height of this image (as a positive number)? Number Is this image upright or inverted with respect to the firefly? O Upright O Inverted

Explanation / Answer

for first lens:

f1 = 6.65 cm

object distance, p1 = 10.5 cm

1/f1 = 1/p1 + 1/q1

q1 = 18.1 cm

now this image will work as object for lens B.

p2 = 61.9 - 18.1 = 43.8 cm

f2 = 27.3 cm

1/f2 = 1/p2 + 1/q2

q2 = 72.6 cm

1. Real

2. d = 72.6 cm

3. m = (q2/p2)(q1/p1) = 2.86

hi = m ho = 2.86 x 5.87 = 16.8 mm  

4. m is +ve so upright

Upright

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