Question

An electromagnetic wave at 100 Hz can penetrate to a depth of 100 m. Then, an electromagnetic wave of 400 Hz will penetrate to a depth of _________m.

Answer 1

The penetration depth

$$\delta=\sqrt{\frac{1}{\pi\mu f\sigma}}$$

Hence,

$$\delta_2=\delta_1\sqrt{\frac{f_1}{f_2}}\\ =100\:\rm m\sqrt{\frac{100\: Hz}{400\: Hz}}=50\:\rm m$$