'''Fiat lux''' veya ''Işık olsun'', ışığın yaratılışını tanımlayan Latince cümle. Aslında Eski Ahit'in Yaratılış bölümünün başında geçen bu ifade Yunanca γενηθήτω φώς (genēthētō phōs)'un adaptasyonudur ki bu Yunanca ifadenin kendisi İbranice aslı יְהִי אוֹר (yhiy 'owr)'dan gelmektedir. İngilizce'de ''Let there be light'' olarak geçer ve bu dilde çok ünlü bir deyiş haline gelmiştir; özellikle de Kral James İncili üçüncü dizesinde var olduğu için. 

Bu ifadenin bulunduğu kısım da, Eski Ahit'teki Yaratılış'ın ilk bölümünün üçüncü dizesidir, Türkçe meâli şöyledir:

:Yar.1: 1 Başlangıçta Tanrı göğü ve yeri yarattı. 

:Yar.1: 2 Yer boştu, yeryüzü şekilleri yoktu; engin karanlıklarla kaplıydı. Tanrı'nın Ruhu suların üzerinde dalgalanıyordu. 

:Yar.1: 3 Tanrı, "Işık ol" dedi ve ışık oldu. 

:Yar.1: 4 Tanrı ışığın iyi olduğunu gördü ve onu karanlıktan ayırdı.[http://www.incil.info/incil-eskiceviri/Yar.htm]

Aynı parçanın Latince'si ise şöyledir:

'''Let there be light''' is an English translation of the Hebrew '''יְהִי אוֹר''' (or ''yehiy 'or'').  Other translations of the same phrase include the Greek phrase '''γενηθήτω φώς''' (or ''genēthētō phōs'').

The phrase comes from the third verse of the book of Genesis in the King James Bible:

:1:1 - In the beginning God created the heaven and the earth. 
:1:2 - And the earth was without form, and void; and darkness was upon the face of the deep.  And the Spirit of God moved upon the face of the waters. 
:1:3 - And God said, Let there be light: and there was light. 
:1:4 - And God saw the light, that it was good: and God divided the light from the darkness.

==Fiat lux==
The Latin phrase '''fiat lux''', from the Latin Vulgate Bible, is typically translated as "let there be light" when relating to Genesis 1:3 (Hebrew: "יְהִי אוֹר"). The full phrase is "''dixitque Deus fiat lux et facta est lux''" ("And said God let there be light, and the light is made"), from the Greek "και είπεν ο Θεός γενηθήτω φως και εγένετο φως" (or ''kai eipen ho Theos genēthētō phōs kai egeneto phōs''), from the Hebrew "וַיֹּאמֶר אֱלֹהִים, יְהִי אוֹר; וַיְהִי-אוֹר" (or ''vayo'mer 'Elohiym yehiy 'or vayehiy 'or'').

Since ''fiat lux'' would be more literally translated as "let light be made" (''fiat'' is from ''fieri'', the passive form of the verb ''facere'', "to make" or "to do"), an alternative Latinization of the original Greek and Hebrew, ''lux sit'' ("light - let it exist" or "let light exist") has been used occasionally, such as in the motto of the University of Washington.

==Use==
Fiat lux is the motto of, and appears on the seals of nine educational institutions: Alfred University , Green Mountain College, Hiram College, University of California, Angelo State University, University of Akron, Union County College in Cranford, NJ, Rollins College, University of Washington and branches in Washington, and Albion College. It is also the motto of the University of Lethbridge in southern Alberta, Canada; Clark University in Worcester, Massachusetts; [Waynesburg University] in Waynesburg, Pennsylvania and Alfred University, Alfred, New York and Mayo College, Ajmer (in India).It has been referred to in multiple movies, books, and games&mdash;often for its metaphorical meaning of dispelling ignorance.  It appears on the outside of Kerns Religious Life Center at Capital University in Columbus, Ohio. Fiat Lux is also on the seal of the University of Liverpool in England, as well as Jacksonville University. It is also the phrase on the seal of the Walter and Eliza Hall Institute of Medical Research, located in Melbourne, Australia. The second half of the same verse, ''Et facta est lux'' appears on the seal of Morehouse College.

Fiat Lux is the name of a controversial cult of some 500 members, based in Switzerland and Germany, led by the prophetess "Uriella". It is relatively unknown outside of German-speaking countries.

Fiat Lux is a Brazilian matchstick trademark.

''Fiat Lux'' is a film recording the pilgrimage of the World Peace Flame in the footsteps of St. Francis from Assisi to Rome. Carried by Mansukh Patel, Lalita Doerstel, Andrew Wells and Savitri MacCuish, the pilgrimage concludes when the World Peace Flame is presented to HH Pope John Paul II.

Fiat Lux is the name of a collection of photographs of the University of California campuses taken by American photographer Ansel Adams.

The [http://www.knightlyorderofthefiatlux.org Knightly Order of the Fiat Lux] is a fraternal non-profit organization dedicated to serving  special needs children through interactive education, living history demonstration, fundraising and volunteering.

In ''Buffy the Vampire Slayer'', Willow Rosenberg uses fiat lux as a spell to create light, designed to help Buffy defeat vampires.

Fiat Lux was a New Wave band formed in Yorkshire in 1982.

"''Let there be light''" is the name of a song on The Songs of Distant Earth''.

"Let There Be Light" is a song on Hip Hop Is Dead, the 2006 album by the rapper NaS.