From the concept of optics on a curvature of a spherical mirror, the proportion for which the focal length is equivalent to half the radius of curvature is fulfilled. Mathematically this is
![f = (R)/(2)](https://img.qammunity.org/2021/formulas/physics/college/j1jcds6k2aofqlys10wuat0pcs2w0za5r2.png)
Here,
f = Focal Length
R = Radius
Rearranging to find the radius we have,
![R = 2f](https://img.qammunity.org/2021/formulas/physics/college/990024t5ruvpbwmxog52zijx6ml1kqe610.png)
Replacing with our values,
R = 2(13.8cm)
R = 27.6cm
Therefore the radius of the spherical surface from which the mirror was made is 27.6cm