Question:
What is the focal length of a convex lens of focal length 30cm in contact with a concave lens of focal length 20cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
Answer:
Focal length of the convex lens (f1) = 30 cm
Focal length of the concave lens (f2) = – 20 cm
Focal length of the system of lenses = f
The equivalent focal length of a system of two lenses in contact
Then,
1/f = 1/ f1 + 1/ f2
1/f = 1/30 -1/20 = 2-3/60 = -1/60
f = -60 cm
Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the lenses acts as a diverging lens.
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