Question

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 lens

The 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 1

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.