Wave-particle Duality

A photon is a particle of light

People, like Newton, favored the idea that light was a particle.

This was until Young, a person, performed an experiment called the double slit experiment which showed light interacting like a wave.

Maxwell, another person, showed that light is an electromagnetic wave — an oscillation of the electric and magnetic fields themselves.

Einstein, another person, then explained the photoelectric effect, showing light also behaves as a particle, so light has both wave AND particle character.

Present day, the majority of people concur that light is a wave and particle simultaneously.

This means that photons can have a “state.” These can be represented as kets!

Sometimes we observe photons having a “spin” — it’s not really spinning, but physicists use the word for an intrinsic angular momentum that particles carry. Photons have spin 1; for photons this shows up as polarization (e.g. circular polarization corresponds to the two helicity state).