Question

Focal length of the plano-convex lens is when its radius of curvature of the surface is
R and n is the refractive index of the lens.

Answer 1

The lens maker's formula is:
$\frac{1}{f}=({\mu }_r-1)(\frac{1}{R_1}-\frac{1}{R_2})$

Here: ${\mu }_r=n{R}_1=R{R}_2=\infty$
Thus:$\frac{1}{f}=(n-1)(\frac{1}{R}-\frac{1}{-\infty })\frac{1}{f}=(n-1)(\frac{1}{R}+0)f=\frac{R}{n-1}$