Question

Help please number 8 an 9

Answer 1

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 $f=10-10^9 Hz$, while X-rays have typical frequencies of $10^{16} - 10^{19} Hz$. Since the energy of a wave is given by
$E=hf$
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 $10^{20}Hz$).