Lay Cables at least cost

Fermat Least-Time Light Refraction Law:
sin A1 / sin A2 = v1 / v2
v1, v2 = speed of light in medium 1 & 2, resp.
A1, A2 = refraction angles in medium 1 & 2 , resp.

To lay cables underground & sea with cost/km = a, b resp.
(b>a)
=> Least-cost if satisfies Fermat Law.