Luminosity of a Spherical Black Body b. Flux Density c. 2 Nature of EMR Light may be regarded as a flood of particles, called photons , or as a wave. In either case, it carries energy through a vacuum at a velocity which is a universal physical constant, and is the same for all observers and for all colors. Light also behaves like a wave in many contexts.
All colors propagate with the same velocity c in vacuo.
Wavelength, frequency and photon energy are all interrelated. These processes are more fully discussed later. A black body is an object that completely absorbs all radiation incident on it, whatever the wavelength. The utility of the black body comes from the relatively simple form of the quantitative description of its radiation. At any wavelength , the number of ergs per second radiated by a square cm black body depend only on temperature T, and may be calculated from a formula called the black-body curve.