Light

A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.[1] Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).[2][3] This wavelength means a frequency range of roughly 430–750 terahertz (THz).

Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli Fantina, Sicily

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

The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants 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 physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[5][6] In this sense, gamma rays, X-rays, microwaves and radio waves 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 quanta 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 wave function collapse. This dual wave-like and particle-like nature of light is known as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

Electromagnetic spectrum and visible light

The electromagnetic spectrum, with the visible portion highlighted

Generally, EM radiation (the designation "radiation" excludes static electric, magnetic, and near fields), or EMR, is classified by wavelength into radio waves, microwaves, infrared, the light, ultraviolet, X-rays, and gamma rays.

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 quanta (called photons) 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 retinal 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. Infrared sensing in snakes depends on a kind of natural thermal imaging, 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 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina 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[7][8] to as broadly as 380–800 nm.[9][10] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm;[11] children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm.[12][13][14]

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

Linear visible spectrum.svg
Other Languages
Afrikaans: Lig
Alemannisch: Licht
አማርኛ: ብርሃን
العربية: ضوء
aragonés: Luz
armãneashti: Luńinâ
অসমীয়া: পোহৰ
asturianu: Lluz
Avañe'ẽ: Mba'erendy
Aymar aru: Qhana
azərbaycanca: İşıq
تۆرکجه: ایشیق
বাংলা: আলো
Bân-lâm-gú: Kng
башҡортса: Яҡтылыҡ
беларуская: Святло
беларуская (тарашкевіца)‎: Сьвятло
български: Светлина
Boarisch: Liacht
bosanski: Svjetlost
brezhoneg: Gouloù
català: Llum
čeština: Světlo
chiShona: Chiedza
Cymraeg: Goleuni
dansk: Lys
Deitsch: Licht
Deutsch: Licht
eesti: Valgus
Ελληνικά: Φως
español: Luz
Esperanto: Lumo
euskara: Argi
فارسی: نور
Fiji Hindi: Prakaas
français: Lumière
Frysk: Ljocht
Gaeilge: Solas
galego: Luz
ГӀалгӀай: Сердало
贛語:
ગુજરાતી: પ્રકાશ
한국어:
հայերեն: Լույս
हिन्दी: प्रकाश
hrvatski: Svjetlost
Ido: Lumo
Ilokano: Lawag
Bahasa Indonesia: Cahaya
interlingua: Lumine
íslenska: Ljós
italiano: Luce
עברית: אור
Basa Jawa: Cahya
ಕನ್ನಡ: ಬೆಳಕು
ქართული: სინათლე
қазақша: Жарық
Kiswahili: Nuru
Kreyòl ayisyen: Limyè
Кыргызча: Жарык
Latina: Lux
latviešu: Gaisma
Lëtzebuergesch: Liicht
lietuvių: Šviesa
Ligure: Luxe
Limburgs: Lèch
la .lojban.: gusni
Luganda: Ekitangaala
lumbaart: Lüs
magyar: Fény
македонски: Светлина
Malagasy: Fahazavana
മലയാളം: പ്രകാശം
मराठी: प्रकाश
مصرى: النور
Bahasa Melayu: Cahaya
Mìng-dĕ̤ng-ngṳ̄: Guŏng
Mirandés: Luç
монгол: Гэрэл
မြန်မာဘာသာ: အလင်း
Nāhuatl: Tlāhuīlli
Nederlands: Licht
Nedersaksies: Locht (straoling)
नेपाली: प्रकाश
नेपाल भाषा: जः
日本語:
Napulitano: Luce
Nordfriisk: Laacht
norsk: Lys
norsk nynorsk: Lys
Nouormand: Lumyire
Novial: Lume
occitan: Lutz
ଓଡ଼ିଆ: ଆଲୋକ
Oromoo: Ifa
oʻzbekcha/ўзбекча: Yorugʻlik
ਪੰਜਾਬੀ: ਪ੍ਰਕਾਸ਼
پنجابی: چانݨ
Patois: Lait
Piemontèis: Lus
polski: Światło
português: Luz
română: Lumină
Runa Simi: Achkiy
русиньскый: Світло
русский: Свет
ᱥᱟᱱᱛᱟᱲᱤ: ᱢᱟᱨᱥᱟᱞ
संस्कृतम्: द्युतिशक्तिः
Scots: Licht
Seeltersk: Lucht
shqip: Drita
sicilianu: Luci
සිංහල: ආලෝකය
Simple English: Light
سنڌي: روشني
slovenčina: Viditeľné svetlo
slovenščina: Svetloba
Soomaaliga: Ileys
کوردی: ڕووناکی
српски / srpski: Светлост
srpskohrvatski / српскохрватски: Svjetlost
Basa Sunda: Cahya
suomi: Valo
svenska: Ljus
Tagalog: Liwanag
தமிழ்: ஒளி
Taqbaylit: Tafat
татарча/tatarça: Яктылык
తెలుగు: కాంతి
ไทย: แสง
ᏣᎳᎩ: ᎢᎦᎢ
Türkçe: Işık
українська: Світло
اردو: روشنی
vèneto: Łuxe
Tiếng Việt: Ánh sáng
Võro: Valgus
walon: Loumire
文言:
Winaray: Lamrag
吴语:
Xitsonga: Rivoningo
ייִדיש: ליכט
Yorùbá: Ìmọ́lẹ̀
粵語:
žemaitėška: Švėisa
中文: