Let `f_(v)` and `f_(r)` are the focal lengths of a convex lens for violet and red lights respectively. If `F_(v)` and `F_(r)` are the focal lengths of a concave lens for violet and red light respectively, then

Options

A. `f_(v) lt f_(r) " and " F_(v) gt F_(r)`
B. `f_(v) lt f_(R) " and " F_(v) lt F_(r)`
C. `f_(c) gt f_(r) " and " F_(v) gt F_(r)`
D. `f_(v) gt f_(r) " and " F_(v) lt F_(r)`

Answer

B

Explanation

`f prop (1)/(n ) " " [ because (1)/(f) (n - 1) ((1)/(R_(1)) - (1)/(R_(2))) ] `
Since, `n_(Red) lt n_(Violet) `
Hence, `f_(r) gt f_(v) and F_(r ) gt F_(v)`.
For comparison we do not use sign conventions.