An object is located 40 centimeters from the first of two thin converging lenses of focal lengths 20 cm and 10 cm, respectively, as shown in figure above. The lenses are separated by 30 cm. The final image formed by the two lenses is located
A. 5.0 cm to the right of the second lens
B. 13.3 cm to the right of the second lens
C. infinitely to the right of the second lens
D. 13.3 cm to the left of the second lens
E. 100 cm to left right of the second lens
(GR0177 #11)
Solution:
1/S + 1/S' = 1/f
For the first lens:
1/S1 + 1/S1' = 1/f1
1/40 + 1/S1' = 1/20
S1' = 40 cm
The image is located 40 cm behind (to the right of) the first lens, which is 10 cm behind the second lens. Therefore, S2 = −10.
For the second lens:
1/S2 + 1/S2' = 1/f2
1/ (−10) + 1/S2' = 1/10
S2' = 5 cm
The final image is located 5 cm to the right of the second lens.
Answer: A
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