What is Light?
Generally when astronomers talk about “light” they mean visible light to which our human eyes are sensitive and that produces a visual sensation. A more general term is Electro-Magnetic radiation (or EM-radiation) that refers not only to visible light, but to everything from Radio transmission up to Gamma radiation. One problem with a definition of this “EM-radiation” is that no scientist can give one model or theory that fully describes it.
Since the early 19th century James Maxwell developed an electro-magnetic field theory that considers light as a wave phenomenon, one that we can imagine when we look at waves and ripples on a water surface or in an oscillating string.
But Einstein demonstrated about one hundred years later that light also behaves in ways that cannot be described by a wave theory. Together with his contemporary Max Planck, he formulated the concept of the photon as a fundamental quantum of energy. Often this is confusingly referred to as a “particle” model, but photons are not “mini-jelly beans” that travel through space, but rather very small amounts (quanta) of energy.
These two interpretations of light, the wave model and the quantum model, are mutually exclusive . This is referred to as Wave-Particle Duality. One can use only one model at any time, depending on which characteristics of light one is studying. Probably the safest general term one could use for this phenomenon is “Energy radiation”, but very generally the term EM-radiation is used, also in astronomy.