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Asked in GSEB Board July 2023 · Combinations and silvered lenses
Given: a convex lens of f₁=+20 cm in contact with a concave lens of f₂=-10 cm.
Idea: lenses in contact add their powers, 1/F=1/(f₁)+1/(f₂), and a concave lens carries a negative focal length.
1/F=1/(20)+1/(-10)=(1-2)/(20)=-1/(20)
F=-20 cm
The concave lens is the stronger of the two — 10 D against 5 D — so the pair diverges, and the negative sign says so.
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