A 6.25-mm-high firefly sits on the axis of, and 11.5 cm in front of, the thin le
ID: 1466897 • Letter: A
Question
A 6.25-mm-high firefly sits on the axis of, and 11.5 cm in front of, the thin lens A, whose focal length is 5.49 cm. Behind lens A there is another thin lens, lens B, with focal length 24.5 cm. The two lenses share a common axis and are 62.3 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
o1 = 6.25 mm
u1 = 11.5 cm
f1 = 5.49 cm
f2 = 24.5 cm
L = 62.3 cm
By Lens equation
(1/f1) = (1/u1) + (1/v1)
Substituting values for f1 and u1 , we get v1 = 10.50 cm
magnification of lens A, m1 = v1 / u1 = 0.913
also m1 = i1 / o1 => i1 = m1o1 => i1 = 5.71 mm
u2 = L - v1 = 51.8 cm
o2 = i1 = 5.71 mm
By Lens equation
(1/f2) = (1/u2) + (1/v2)
substituting values for f2 and u2, we get v2 = 46.49 cm
magnification of lens B, m2 = v2 / u2 = 0.897
Also m2 = i2/o2 => i2 = m2o2 => i2 = 5.12 mm
(A) Real
(B) v2 = 46.49 cm
(C) i2 = 5.12 mm
(D) Upright