The Avestan word raocah-
Word raocah-
Meaning light
See also Homeric Greek: λευκός ‘light, bright, brilliant’
Proto-Indo-European: *leukós ‘light, bright, clear’

Other words that might be of interest
təmah- ‘darkness’, hvarə ‘sun’, axšaēna ‘black, dark’, ātar- ‘fire’, huuar- ‘sun’