In 1900, Planck (a German physicist who lived from 1858 to 1947) announced oscillating electrons radiate electromagnetic waves with intermittent energy. Before that, it was thought that electromagnetic energy fluctuated continuously and could be endlessly split into smaller and smaller parts. According to Planck, energy is emitted in proportion to oscillation frequency. This proportionality constant is called "Planck's constant" (h = 6.6260755 x 10-34), and oscillation frequency times Planck's constant is known as an "energy quantum."

If we try viewing this as light particles, we can consider electromagnetic waves of a certain oscillation frequency to be a group of photons with energy equal to oscillation frequency times Planck's constant. Photons are zero-mass particles, but because they have energy, they also possess momentum.