*leuk-

The core idea of this root was:

light, bright, shining

It’s the origin of the word light in the sense of lightning and of the Latin branch around luce with words like:

• illuminate
• luminous
• illustrate 
• lucent, lucid, translucent 
• lunar
• lunacy 
• Lucifer

Also the Greek leuko- belongs to the family and origially meant “clear, white”.