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According to the thin lens formula, which one of the following is true regarding the focal length of the lens?(a) f is positive for concave lens(b) f is negative for convex lens(c) f is positive for a diverging lens(d) f is negative for concave lensThe question was posed to me in an internship interview.My enquiry is from Ray Optics topic in portion Ray Optics and Optical Instruments of Physics – Class 12

Answer»

The correct answer is (d) f is negative for CONCAVE lens

Explanation: Thin lens formula is given as:

\(\FRAC {1}{f}=\frac {1}{V}-\frac {1}{u}\)

Where u = distance of the object from the optical center of the lens; v = distance of the IMAGE from the optical center of the lens; f = FOCAL length of a lens. According to the thin lens formula, f is positive for converging or convex lens and f is negative for a diverging or concave lens.



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