light

Electromagnetic waves (made up of electric and magnetic components) in the visible range, having a wavelength from about 400 nanometres in the extreme violet to about 700 nanometres in the extreme red. Light is considered to exhibit particle and wave properties, and the fundamental particle, or quantum, of light is called the photon. A light wave comprises two transverse waves of electric and magnetic fields travelling at right angles to each other, and as such is a form of electromagnetic radiation. The speed of light (and of all electromagnetic radiation) in a vacuum is approximately 300,000 km/186,000 mi per second, and is a universal constant denoted by c.

Light is a form of energy that is mainly visible to the human eye. Light is radiated by hot objects such as the Sun or an electric light bulb.

The nature of light travelling through a medium can be explained with the help of Young's double-slit experiment. A double slit is placed between a light source and a screen. Light diffracted by the double slit forms a pattern of bright and dark bands (called fringes) on the screen. The fringe pattern is due to the light waves moving through the slits and being diffracted. At a point where light waves are in phase (a crest and crest meeting at the same point) a constructive wave front is formed with a larger amplitude than that of the two waves forming the constructive wave front. This is seen as a bright band on the screen. At a point where the light waves are out of phase (a crest and a trough meeting at the same point) a destructive wave front is formed with zero amplitude (the waves effectively cancel each other out). This is seen as a dark band on the screen.