Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. Light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics. There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 40% of sunlight is visible), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames.
Source: en.wikipedia.org/wiki/Light - 25.03.2010
Light is not tangible, not even viewable, yet it transforms the appearance of everything it touches.