|is a photon a specific unit of measurement of light? Has it been decided if light is a particle or a wave? Why? — J, Australia
There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave—for example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle—for example, it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light, the smallest packet of light that can exist. You can't have half a photon of light—it's all or nothing. The amount of energy in a particular photon of light depends on the frequency (or wavelength) of that light.