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
![f=10-10^9 Hz](https://tex.z-dn.net/?f=f%3D10-10%5E9%20Hz)
, while X-rays have typical frequencies of
![10^{16} - 10^{19} Hz](https://tex.z-dn.net/?f=10%5E%7B16%7D%20-%2010%5E%7B19%7D%20Hz)
. Since the energy of a wave is given by
![E=hf](https://tex.z-dn.net/?f=E%3Dhf)
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
![10^{20}Hz](https://tex.z-dn.net/?f=10%5E%7B20%7DHz)
).