Question

The radii of curvature of lens are $+20cm$ and $+30cm$. The material of the lens has refracting index $1.6$. find the focal length of the lens (a) if it is placed in air, and (b) if it is placed in water $(\mu =1.33)$

Answer 1

$R_1=+20cm;R_2=+30cm;\mu =1.6$

a) If placed in air:

$\frac{1}{f}=({\mu }_g-1)(\frac{1}{R_1}-\frac{1}{R_2})=(\frac{1.6}{1}-1)(\frac{1}{20}-\frac{1}{30})$

$\Rightarrow f=60/6=100cm$

b) If placed in water:

$\frac{1}{f}=({\mu }_w-1)(\frac{1}{R_1}-\frac{1}{R_2})=(\frac{1.6}{1.33}-1)(\frac{1}{20}-\frac{1}{30})$

$\Rightarrow f=300cm$