A plano convex lens is made of material having refractive index 1.8. If the radius of curvature of the curved surface is 80 cm, then focal length of the lens is:

100 cm

The correct answer is Option (1) → 100 cm

Using Len's maker formula,

$\frac{1}{F} = (\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

[μ = Refractive index]

[f = focal length]

$= (1.8-1)\left(\frac{1}{80}-\frac{1}{\infty}\right)$

$=0.8×\frac{1}{80}=\frac{1}{100}$

$\Rightarrow f = 100cm$