1. A 5.59-mm-high firefly sits on the axis of, and 11.3 cm in front of, the thin

Question

1. A 5.59-mm-high firefly sits on the axis of, and 11.3 cm in front of, the thin lens A, whose focal length is 6.23 cm. Behind lens A there is another thin lens, lens B, with focal length 25.9 cm. The two lenses share a common axis and are 59.5 cm apart. Is the image of the firefly that lens B forms real or virtual? 2. How far from lens B is this image located (expressed as a positive number)? 3. What is the height of this image (as a positive number)? 4. Is this image upright or inverted with respect to the firefly?

Explanation / Answer

(1)

from the thin lens eqution

1/f = 1/p + 1/q

1/6.23 = 1/11.3 + 1/q

q = 13.88 cm

Since the lenses are 59.5cm apart, this image is the object for the second lens and 59.5 - 13.88 = 45.61cm in front of it

Then for the second lens

1/25.9 = 1/45.61+ 1/q

q = 59.92 cm

(b)

M = ( -13.88/11.3) ( -59.92/45.61) =1.61

h' = (1.61)(5.59) = 8.99mm

Since the overall magnification is positive, the image is upright and real

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