"why is 21-cm radiation the best way to map the spiral arms in the milky way? "

The best way to map the spiral arms in the Milky Way mainly is by using the 21 cm line radiation is because these photons have the ability to penetrate the dense clouds of gas and dust in the disk. The 21 cm line radiation, also called the hydrogen line radiation is the electromagnetic radiation spectral line that is produced by a change in the energy state of neutral hydrogen atoms, the most abundant substance in space. The actual wavelength of the 21 cm line radiation is 21.1 centimeters, and it has a frequency of 1420.4 MHz.

its c i just checked it

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.