Light is a form of electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye. To understand light, we need to understand electromagnetic radiation.

According to the wave–particle duality theory, electromagnetic radiation has a dual nature as both particles and waves. One way to look at it is as changing electric and magnetic fields which propagate through space, forming an electromagnetic wave. This wave has amplitude, which is the brightness of the light, wavelength, which is the color of the light, and an angle at which it is vibrating, called polarization. This was the classical interpretation, crystallized in Maxwell's Equations, which held sway until Planck, Einstein and others came along with quantum theory. In terms of the modern quantum theory, electromagnetic radiation consists of particles called photons, which are packets ("quanta") of energy which move at the speed of light. In this particle view of light, the brightness of the light is the number of photons, the color of the light is the energy contained in each photon, and four numbers (X, Y, Z and T) are the polarization.

Which interpretation is correct? Both of them, actually. It turns out electromagnetic radiation can have both wave-like and particle-like properties as demonstrated in experiments such as the dual slit experiment. In this exploration of light, we will primarily take the wave viewpoint as it is a more useful description of the everyday properties of light, but keep in mind that both viewpoints are valid, and sometimes we will use the quantum viewpoint too.
Source(s):
http://en.wikipedia.org/wiki/Light
http://library.thinkquest.org/27356/p_index.htm

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