Final answer:
Nuclear magnetic resonance (NMR) is a resonance phenomenon in which nuclei in a magnetic field absorb and reemit only certain frequencies. The alignment of nuclear spin can be flipped using an external radio frequency signal.
Step-by-step explanation:
Nuclear magnetic resonance (NMR) is a resonance phenomenon in which nuclei in a magnetic field act like resonators that absorb and reemit only certain frequencies. When placed in an external magnetic field, the nuclear spin alignment can be flipped using an external radio frequency signal. The specific frequency of the radio waves that are absorbed and reemitted depends on the type of nucleus, the chemical environment, and the external magnetic field strength.