Question

$f_v$ 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 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)

$f_v<f_R$ and $F_v<F_R$

$\begin{array}{c}\text{Focal length is directly proportional to}\\ \text{wavelength of light passed.}\\ \text{As red light has higher wavelength ,}\\ \text{it refracts less and has greater focal lens}\\ \text{on the contrary, violet light converges}\\ \text{more and has small focal length.}\\ \text{Irrespective of the type of lens.}\\ \text{So.}\\ f_v<f_R\text{and}{F}_v<f_R\\ \text{Option}B\text{is correct.}\end{array}$