Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that the human eye perceives. The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization. Light is a transverse wave and does not need any medium to travel. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of 299,792,458 meters per second. Light can also be described as a stream of photons, massless packets of energy.