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==Sources== ===Lasers=== [[Laser]] light from gas or crystal lasers is highly collimated because it is formed in an [[optical cavity]] between two parallel [[mirror]]s which constrain the light to a path perpendicular to the surfaces of the mirrors.<ref>{{cite web |url=http://www.worldoflasers.com/laserproperties.htm |title=Properties of Lasers |author=<!--Staff writer(s); no by-line.--> |date=2015 |website=World of Lasers |access-date=5 August 2015 }}</ref> In practice, gas lasers can use concave mirrors, flat mirrors, or a combination of both.<ref>{{cite book|title=Engineering Physics|last=Joshi|date=2010|publisher=[[Tata McGraw-Hill Education]]|isbn=9780070704770|page=517}}<!--|access-date=6 August 2015 --></ref><ref>{{cite book|title=Engineering Physics 1: For WBUT|author=<!--Staff writer(s); no by-line.-->|date=n.d.|publisher=Pearson Education India|isbn=9788131755938|location=India|pages=3β9}}<!--|access-date=6 August 2015 --></ref><ref>{{cite book|title=Elementary Modern Physics|last=Tipler|first=Paul|date=1992|publisher=MacMillan|isbn=9780879015695|page=149}}<!--|access-date=6 August 2015 --></ref> The [[Divergence (optics)|divergence]] of high-quality laser beams is commonly less than 1 [[milliradian]] (3.4 [[arcmin]]), and can be much less for large-diameter beams. [[Laser diode]]s emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens. ===Synchrotron light=== [[Synchrotron light]] is very well collimated.<ref>{{cite book|title=Synchrotron Radiation Research|last1=Winick|first1=Herman|last2=Doniach|first2=S|date=2012|publisher=[[Springer Science & Business Media]]|isbn=9781461579984|page=567}}<!--|access-date=6 August 2015 --></ref> It is produced by bending relativistic electrons (i.e. those moving at [[Special relativity|relativistic]] speeds) around a circular track. When the electrons are at relativistic speeds, the resulting radiation is highly collimated, a result which does not occur at lower speeds.<ref>{{cite book|title=Synchrotron Radiation: Basics, Methods, and Applications|last1=Mobilio|first1=Settimio|last2=Boscherini|first2=Federico|last3=Meneghini|first3=Carlo|date=2014|publisher=Springer|isbn=9783642553158|page=31}}<!--|access-date=6 August 2015 --></ref> ===Distant sources=== The light from [[star]]s (other than the [[Sun]]) arrives at Earth precisely collimated, because stars are so far away they present no detectable angular size. However, due to refraction and turbulence in the Earth's atmosphere, starlight arrives slightly uncollimated at the ground with an [[astronomical seeing|apparent angular diameter of about 0.4 arcseconds]]. Direct rays of light from the Sun arrive at the Earth uncollimated by one-half degree, this being the [[angular diameter]] of the Sun as seen from Earth. During a [[solar eclipse]], the Sun's light becomes increasingly collimated as the visible surface shrinks to a thin crescent and ultimately a [[Baily's beads|small point]], producing the phenomena of distinct shadows and [[shadow bands]]. ===Lenses and mirrors=== [[Image:CollimatingLensSVG.svg|thumb|150px|right|An example of an optical collimating lens.]]A perfect [[parabolic mirror]] will bring parallel rays to a focus at a single point. Conversely, a point source at the focus of a parabolic mirror will produce a beam of collimated light creating a [[collimator]]. Since the source needs to be small, such an optical system cannot produce much optical power. [[Spherical mirror]]s are easier to make than parabolic mirrors and they are often used to produce approximately collimated light. Many types of [[lens (optics)|lens]]es can also produce collimated light from point-like sources.
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