For \(27 \mathrm{MHz}\) radio waves, the wave intensity has been reduced by a factor of 3 at a depth of approximately \(15 \mathrm{~cm}\). At this point in the tissue, the electric field amplitude has decreased by a factor of

A. 9

B. \(3 \sqrt{3} \cap\)

C. 3

D. \(\sqrt{3}\)

Radio waves and microwaves are used in therapy to provide "deep heating" of tissue because the waves penetrate beneath the surface of the body and deposit energy. We define the penetration depth as the depth at which the wave intensity has decreased to \(37 \%\) of its value at the surface. The penetration depth is \(15 \mathrm{~cm}\) for \(27 \mathrm{MHz}\) radio waves. For radio frequencies such as this, the penetration depth is proportional to \(\sqrt{\lambda}\), the square root of the wavelength.