of electromagnetic radiation, light. Light we experience in everyday life is
in fact a 'swarm'
of electromagnetic quanta i.e. Photons. Each photon carries a quantum of energy
which is equal to planck's constant times its frequency. Photon frequency is
nontrivially related to color we observe if it is within a visible part of
the electromagnetic radiation spectrum. X-ray photons for example are not
visible. Due to their high energy,
they can penetrate through matter.