Light


 * Visible light** (commonly referred to simply as **light**) is electro magnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometers to about 740 nm. The visible light range is located between infrared and ultraviolet light, both of which are not visible to human sight; infrared's wavelength is too long for humans to see, and ultraviolet's light is too short.

Light is emitted by electrons; the color of light is determined by the atom that the electron is bound to. Different atoms omit different frequencies from the electrons. Some properties of visible light include; intensity, frequency, and wave length. Wavelength is the distance between troughs (the valley of the wave being measured) or crests (the top of the wave being measured). Frequency is how often the wave goes through a cycle. Therefore, frequency and wavelength have an inverse relationship - as frequency increases, wavelength decreases or vice versa. The equation for frequency is as follows, where f is frequency, v is velocity, and lambda is wavelength: The study of light is known as optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light. Refraction is the bending of light rays when passing through a surface between one transparent material and another.



Speed of visible light
The speed of light in a [|vacuum] is defined to be exactly 299,792,458 [|m/s] (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation are believed to move at exactly this same speed in vacuum. Different [|physicists] have attempted to measure the speed of light throughout history. [|Galileo] attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by [|Ole Rømer], a Danish physicist, in 1676. Using a [|telescope], Rømer observed the motions of [|Jupiter] and one of its [|moons] , [|Io]. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of [|Earth] 's orbit. [|[4]] Unfortunately, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Refraction
Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by [|Snell's Law] :

where is the angle between the ray and the [|surface normal] in the first medium,  is the angle between the ray and the surface normal in the second medium, and n1 and n2 are the [|indices of refraction], //n// = 1 in a [|vacuum] and //n// > 1 in a [|transparent] [|substance]. When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not [|orthogonal] (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as [|refraction]. The refractive quality of [|lenses] is frequently used to manipulate light in order to change the apparent size of images. [|Magnifying glasses], [|spectacles] , [|contact lenses] , [|microscopes] and [|refracting telescopes] are all examples of this manipulation.

Light pressure
Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by //[|c]//, the speed of light. Due to the magnitude of //c//, the effect of light pressure is negligible for everyday objects. For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U. S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers. However, in nanometer -scale applications such as NEMS, the effect of light pressure is more significant, and exploiting light pressure to drive NEMS mechanisms and to flip nanometer-scale physical switches in integrated circuits is an active area of research