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What sign does the focal length of a diverging lens carry, by the stated convention?
AIt always carries a positive sign
BIt always carries a negative sign
CIt carries no defined sign at all
DIt depends on the colour of the light
Answer & Solution
Correct answer: B. It always carries a negative sign
1. The focal length of a diverging, or concave, lens is defined to be negative.
2. A converging, or convex, lens instead carries a positive focal length under the same convention.
3. The sign follows from the lens type, not from the colour of light passing through it.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.3 Lenses_
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