Question

The diameter of aperature of a plano convex lens is 6 cm and its maximum thickenss is 3 mm. If the velocity of light in the material of lens is $2\times {10}^{8}m{s}^{-1}$, calculate the focal length of the lens

• A. 10 cm
• B. 15 cm
• C. 20 cm
• D. 30 cm

Answer 1

The correct option is D 30 cm



$t=0.3cm$ (thickness
$(2R-t))t=y^2\Rightarrow 2RT-t^2=y^2$
As $t<R;2RT\simeq y^2$
and $R=\frac{y^2}{2t}=\frac{3^2}{2\times 0.3}=15cm\mu =\frac{3\times {10}^8}{2\times {10}^8}=1.5$
Also, $\frac{1}{f}=(\mu -1)[\frac{1}{R_1}-\frac{1}{R_2}]=(1.5-1)[\frac{1}{15}-\frac{1}{8}]30cm$