Question

If $ f_ v$ and $ f_R $ are the focal lengths of a convex lens for violet and red light respectively and $ F_v $ and $ F_R $ are the focal lengths of a concave lens for violet and red light respectively, then we must have

• A. $ f_V > f_ R$ and $F_v > F_ R$
• B. $ f_V < f_ R $ and $ F_v > F_ R$
• C. $ f_V > f_ R $ and $ F_v < F_ R$
• D. $ f_V < f_ R $ and $ F_v < F_ R$

Answer 1

Answer: (B)

F or a convex lens, $ f_R > F_ V \, \, or \, \, f_V < f_R $ For a
concave lens, focal length is negative.
$\therefore \, \, |F_V| < |F_R | \, \, or \, \, F_v > F_ R $ as the smaller negative
value is bigger