The Proto-Indo-European word *leuk-; lóuk-o-
Word *leuk-; lóuk-o-
Meaning bright light
See also Homeric Greek: λευκόν ‘white’
Homeric Greek: λευκόω ‘whiten over’
Homeric Greek: λευκαίνω ‘make white’
Linear B: re-u-ko-nu-ko ‘(decorated) with white nails’
Hittite: lu-uk-ta ‘to get light, to light up, to dawn’

Comments
OCS luci, Russ. lug, Skr. rocate, Gk. leukos, Eng. lēoht/light, Gm. lioht/licht, Kamviri luka, Goth. liuhaþ, ON leygr, Toch. luk, Oscan Lúvkis, Umbrian Vuvçis, Welsh llug, Ir. loscaim/loiscim, Lat. lux, Lith. lauka, Ltv. lauks, Hitt. lalukkes, Lyc. luga, Luw. luha-, Arm. loys, Kamviri ṛ uč, Gaul. leux, Av. raočant

Other words that might be of interest
*ar(e)-ĝ- ‘glittering, bright, white, fast’, *k̂euk- ‘burn, to shine, bright’, *dheu-1 ‘dust, smoke, vapor’, *bʰel- ‘light, bright’, *bʰer- ‘brown, shining’, *dʰel- ‘light’, *dhuh₂mós ‘smoke’, *gʷʰh₂í- ‘bright, shining’, *h₂éhₓtr̥ ‘fire’, *h₁égni- ‘fire’, *h₃és-no- h₃és-i- ‘ash’, *meldh- ‘lightning’, *sneudh- ‘cloud, mist’, *séhₐul ‘sun’, *temó- ‘dark’, *tón-r̥- ‘thunder’, *reudh- ‘light red’, *kwapor ‘smoke, steam, heat’, *péh₂ur ‘fire’, *h₂eidh- ‘kindle, ignite’, *bʰéh₂(e)s- ‘light’, *bʰéh₂tis ‘light’, *bʰel- ‘shine’, *bʰerhₓĝ- ‘shine, gleam’, *bʰleg- ‘shine, burn’, *dh(o)ngu- ‘dark’, *g(e)ulo- ‘fire, glowing coal’, *ĝher- ‘shine, glow’, *gʷhaidrós ‘bright, shining’, *h₃meigh- ‘drizzle, mist’, *h₄elh₁- ‘burn’, *hₐeidh- ‘burn, fire’, *hₐel- ‘burn’, *(hₐ)mauros ‘dark’, *(hₐ)merhₓgʷ- ‘dark’, *hₐewes- ‘shine’, *hₓn̥gʷnis ‘fire’, *kenhₓis ‘ash’, *kenk- ‘burn’, *ker- ‘burn’, *tómhₓes- ‘dark’, *wer- ‘burn’, *k̂ehₐu- ‘burn’, *kʷap- ‘seethe, smoke’, *k̂seh₁- ‘burn, singe’, *lap- ‘shine’, *leip- ‘light, cause to shine’, *leuk- ‘shine’, *mer- ‘shine, shimmer’, *merk- ‘darken’, *nébʰes- ‘mist, cloud, sky’, *(s)kand- ‘shine, glitter, moon’, *(s)koitrós ‘bright, clear’, *skótos ‘shadow, shade’, *sk̂ōyh̥a ‘shade’, *(s)m(e)ug(h)- ‘smoke’, *(s)plend- ‘shine’, *sweid- ‘shine’, *swer- ‘darken’, *ghromos ‘thunder, groan’, *h₂éhₓōs ‘ash’, *lóuk(es)- ‘light’, *leukós ‘light, bright, clear’, *(s)tenhₓ- ‘groan, thunder’