Light

Light or visible light is within the portion of the that can be by the . in the range of 400–700 (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the (with longer wavelengths) and the (with shorter wavelengths). This wavelength means a range of roughly 430–750 (THz).

The main source of light on Earth is the . provides the that use to create mostly in the form of , which release energy into the living things that digest them. This process of provides virtually all the energy used by living things. Historically, another important source of light for humans has been , from ancient campfires to modern . With the development of and , electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called . For example, use light to locate mates, and use it to hide themselves from prey.

The primary properties of visible light are , propagation direction, frequency or wavelength , and , while its , 299,792,458 meters per second, is one of the fundamental of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in a vacuum.[4]

In , the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, , , and are also light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed. The absorbed energy of the EM waves is called a photon, and represents the of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave instantly collapses to a single location, and this location is where the photon "arrives." This is what is called the . This dual wave-like and particle-like nature of light is known as the . The study of light, known as , is an important research area in modern physics.

Generally, EM radiation (the designation "radiation" excludes static electric, magnetic, and ), or EMR, is classified by wavelength into , , , the that we perceive as light, , , and .

The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

EMR in the visible light region consists of (called ) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule in the human retina, which change triggers the sensation of vision.

There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. depends on a kind of natural , in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it.

Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 and located in the of the human eye cannot detect the very short (below 360 nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses (such as insects and shrimp) are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light.

Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm;

Plant growth is also affected by the color spectrum of light, a process known as .

The speed of light in a is defined to be exactly 299,792,458  (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the meter is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.