Whether it is a tumor or not, Magnetic Resonance Imaging (MRI) can tell. Your head is full of tiny radio transmitters (the nuclear spins of the hydrogen nuclei of your water molecules). In an MRI unit, these little radios can be made to broadcast their positions, giving a detailed picture of the inside of your head.

In this simulation you can:

• Recognize that light can flip spins if the energy of the photons matches the difference between the energies of spin up and spin down.
• Recognize that the difference between the energies of spin up and spin down is proportional to the strength of the applied magnetic field.
• Describe how to put these two ideas together to detect where there is a higher density of spins.