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Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. 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). This wavelength means a frequency range of roughly 430–750 terahertz (THz).
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 polarisation, 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.
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons and 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.
^ CIE (1987). International Lighting Vocabulary. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.
By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.”
^ Pal, G. K.; Pal, Pravati (2001). "chapter 52". Textbook of Practical Physiology (1st ed.). Chennai: Orient Blackswan. p. 387. ISBN 978-81-250-2021-9. Retrieved 11 October 2013. The human eye has the ability to respond to all the wavelengths of light from 400–700 nm. This is called the visible part of the spectrum.
^ Buser, Pierre A.; Imbert, Michel (1992). Vision. MIT Press. p. 50. ISBN 978-0-262-02336-8. Retrieved 11 October 2013. Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.
^ Gregory Hallock Smith (2006). Camera lenses: from box camera to digital. SPIE Press. p. 4. ISBN 978-0-8194-6093-6.
^ Narinder Kumar (2008). Comprehensive Physics XII. Laxmi Publications. p. 1416. ISBN 978-81-7008-592-8.

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  1. gun.deals

    UTG 95lumen Combat Xenon Weapon Light with Integral Mount - $15.70 + FS over $25

    UTG 95lumen Combat Xenon Weapon Light with Integral Mount - $15.70 + FS over $25
  2. Liberty19

    WTS/WTT OR  3 BCM sling mounts, 3 BCM 5 slot nylon rail sections, 1 BCM 1913 light mount

    Sling mounts: $10 each or 3 for $25 Rail sectons: $5 each or 3 for $12 1913 light mount: $20 Everything for $50!
  3. HariKh

    WTS OR  $200 Surefire XC1 weapons light and Bravo light bearing holster glock 19/23/32

    Brand new in box Surefire XC1 weapons light never used. Will include a Bravo Concealment "DOS-L" light bearing inside the waistband holster. $200 OBO located in Eugene Five four one-nine six eight-six six six three
  4. RedneckRampage

    WTS/WTT OR  Light Force 170 Stryker Off-road Lights

    I have 4, Light Force 170 off road lights for sale. All work and are in good condition, 100 watts each. I have 4 clear covers and 2 yellow fog filters/covers. $250 or trade for .45acp ammo.
  5. TJdamon

    WTS OTHER  9" HID off-road light

    1- 9" HID off road light, I had another but I hit a deer and destroyed it. 35w comes with an amber 3000k bulb and a white bulb. Asking $100 obo Damon
  6. gun.deals

    Canwelum Strobe Cree LED Pistol Light - $28.79 shipped

    Canwelum Strobe Cree LED Pistol Light with Weaver Quick Release - $28.79 + Free Shipping
  7. ATCclears


    <broken link removed> Interesting potential.
  8. g.i. joe

    WTS OR  Viking Tactics Light Mount

    New in package, VTAC-MK4, 1 left, Coyote, $15 Open to trades, what do you got? FTF at my office in Tigard, or we can meet in Newberg or McMinnville on my commute , shipping can be arranged on your dime and PP Friends and family.
  9. P

    WTS OR  Browning Light Twelve Belgium

    Up for sale is a well used Browning Light Twelve 12 gauge sporting a 25" Imp. cylinder fixed choke barrel. Not the original barrel, lots of life left in this shooter. Heck of a clay buster and grouse gun. $250
  10. J

    WTS/WTT OR  Walther P99AS compact with TRL3 light

    Almost NIB, fired 50 rounds. Put Talon grips on, added a streamlight TRL3 light, awesome combo. Needed to upgrade to a larger capacity Glock. I will trade for a Glock, AR or $535 cash obo.. Thank you. Jeremy
  11. aslinged

    WTT OR  ADM T1 mount, Troy VFG, Vltor light mount. Trades.

    ***All excellent. 45 OBO shipped for each the grip and light mount. Bipod and sling adapters (both GGG I think), painted are available to throw in with a purchase as a freebie. Check my other ads, I'll discount for multiple items.
  12. NWPatriot

    WTS/WTT WA  Lasermax laser/light combo

    As subject states, Lasermax laser/light combo unit. Also includes momentary on/off pressure switch. Fits Remington 870, Remington 1100, Remington 1187 12-gauge shotguns with 18" or longer barrels & two or more-round extension tubes. (was on Mossberg 930 Spx). All tools, manual, etc. Summary...
  13. gun.deals

    LaserMax Spartan Laser and Light Combo - $79

    LaserMax Spartan Laser and Light Combo - $79 (Free S/H over $99 w/code "57CAMP")
  14. orygun

    Want to mount a light to my Maverick 88

    I've got Maverick 88 with a 20" barrel and a full length magazine (7 round I believe) and I'm looking for a mount to put a light on. I've got a light that fits on a typical rail, but I no got the rail! Anyone have a clamp on mount that they've used and like?
  15. .45's and .38's

    WTS OR  Sling and mount for light

    Up for sale is a isreali rifleman's sling. This sling is bombproof and has mounts that can go on any sling swivel. The light mount is a solid metal mount that goes on a shotgun, rifle barrel. 25 for them both firm. Gresham.
  16. joken

    Hawkeye Ultra Light (Model HM77R)

    I have this rifle in .223. It was built only 2009/2010. Wood stock with black forend cap. Gun is 98% and books for $585. My question is what the heck is it worth? There are none of these for sale and no completed sales on GB. Some guns that are scarce have a RMV much higher than the BB. Thanks, Ken
  17. oregonnash


    I have a used SureFire M900v grip/weapon light in excellent condition. Color is FDE. Uses three CR123A batteries. Mounts to any picatinny rail with ARMS quick detach lever. Has a pressure sensitive grip as well as constant on switch. This model is no longer being manufactured. Asking $160. SOLD
  18. C

    WTS/WTT OR  Ruger LCP 380 tac light and +1 magazines

    Ruger LCP with upgraded hammer spring recoil spring and sweet pee trigger. Two 6 rd magazines with plus one extensions, one 10 rd magazine. Veridian Tactical light that mounts to trigger guard with pocket holster. Lazermax trigger guard mounted red lazer sight. Also has a Hogue grip. I would be...
  19. gun.deals

    NEW! LaserMax Spartan Laser and Light Combo - $79

    URL="NEW! LaserMax Spartan Laser and Light Combo - $79.00 ($5 S/H over $99 w/code "37SPRING")"]NEW! LaserMax Spartan Laser and Light Combo - $79.00 ($5 S/H over $99 w/code "37SPRING")[/URL]