8) The reason is that MRIs use radio-waves, which are electromagnetic waves with lower frequency, and so lower energy, than X-rays.
In fact, typical frequencies for radio waves are
, while X-rays have typical frequencies of
. Since the energy of a wave is given by
where h is the Planck constant and f the frequency, we see that the lower the frequency, the lower the energy. Therefore, radiowaves have less energy than X-rays, so they are less ionising and less dangerous than X-rays.
9) The false statement is:
a) gamma-rays are low-frequency waves.
It's actually the opposite, in fact: gamma rays are the electromagnetic waves with higher frequencies (above
).