Light
AMU stands for Atomic Mass Unit. It is a unit used to measure the mass of atoms and subatomic particles. 1 AMU is approximately equal to the mass of a proton or neutron.
The number of protons in an atom's nucleus, which determines its identity as an element.
The mass (in grams) of one mole of a substance, which is numerically equal to its atomic or molecular weight in AMU.
A range or continuum of all possible wavelengths (or frequencies) electromagnetic radiation can have.