Three lenses, each of focal length 90 cm, are placed in contact. What is the focal length of the combination?
A) 27 cm
B) 93 cm
C) 1/30 cm
D) 45 cm
E) 30 cm

Question

Quizplus · Accepted Answer

When lenses are in contact, the effective focal length is given by:
1/f = 1/f1 + 1/f2 + 1/f3 +...

Since all three lenses have the same focal length of 90 cm, the equation simplifies to:
1/f = 1/90 + 1/90 + 1/90
1/f = 3/90
f = 30 cm

Therefore, the combined focal length of the three lenses is 30 cm, which corresponds to answer choice E.

Three Lenses, Each of Focal Length 90 Cm, Are Placed