Question

A biconvex lens of radii of curvature $20cm$ has refractive index $1.56$ for blue colour and $1.48$ for red colour. The approximate linear spread of focus for white light is :

• A. $3.6cm$
• B. $4.2cm$
• C. $3.0cm$
• D. $5.0cm$

Answer 1

The correct option is C $3.0cm$

$\frac{1}{F}=(n-1)\frac{2}{R}\Rightarrow F=\frac{R}{2(\mu -1)}$

$\Rightarrow \Delta F=F_R-F_B=\frac{R}{2}[\frac{1}{({\mu }_R-1)}-\frac{1}{({\mu }_B-1)}]\Rightarrow \frac{10}{0.48}-\frac{10}{0.56}\cong 3cm$

Hence $\left(C\right)$ is correct.