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:
Answer & explanation
Correct answer: option 1
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]
$= (1.8-1)\left(\frac{1}{80}-\frac{1}{\infty}\right)$
$=0.8×\frac{1}{80}=\frac{1}{100}$
$\Rightarrow f = 100cm$