The depth of penetration of electromagnetic wave in a medium having conductivity $\sigma$ at a frequency of $1 \; \mathrm{MHz}$ is $25 \mathrm{~cm}$. The depth of penetration at a frequency of $4 \; \mathrm{MHz}$ will be
- $6.25 \mathrm{~cm}$
- $12.50 \mathrm{~cm}$
- $50.00 \mathrm{~cm}$
- $100.00 \mathrm{~cm}$