## About[]

Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400–700 nm), or perhaps 380–750 nm. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.

Three primary properties of light are:

- Intensity
- Frequency or wavelength
- Polarization

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. Light ranges from wavelengths of 7x10-5 cm (red) to 4x10-5 cm (violet) and (like all electromagnetic radiation) travels at the speed of light, 299,792,458 meters per second or 186,282 miles per second. (Interesting fact: the speed of light is actually defined to be 299,792,458 meters per second and scientists combine this with the definition of a second to create the definition of a meter! As stated at the 17th General conference on weights and Measures, "The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.")

The frequency (number of wavelengths per second) of a light wave may be calculated using the equation c=ln where l is the wavelength, n is the frequency and c is the speed of light. In quantum theory, a photon has energy equal to hn, where h is Plank's constant and n is the frequency of the light in classical theory.

## Huygens Principle[]

In many cases, light waves are very much like water waves. One distinct difference, however, is that water waves are waves on a 2 dimensional plane (surface of the water) while light waves are waves within 3 dimensional space.

The wave theory, proposed by the Dutch physicist Christian Huygens, viewed light as an impulse moving in all directions. Consider a point P in space. If an impulse starts at P, then the effect of the impulse, after some time, will be equidistant from P in all directions -- one can visualize this impulse as an expanding sphere with center P.

Huygens called this sphere a front. Most importantly, every point on a front can be a source of new wavelets (act just like point P), and the envelope around those wavelets forms another front. In other words, a second front can be created from the first by making each point of the first front the origin of another impulse. All these impulses combine and appear as if the original front is expanding.^{[1]}

## References[]

## Video[]

thumb|300px|right|MIT Vibrations and Waves Fall 2004