Answer: Ionizing radiations have high energy and high frequencies (such s gamma rays and X-rays), whereas non ionizing radiations have low frequencies and low energy (such as radio-waves and micro-waves). It is known that radiation energy is directly proportional to the frequency. So, larger the energy and frequency, larger will be be its penetration power.
Thus, ionizing radiations are able to penetrate through the surfaces wheres non ionizing radiations can't.