actinic ray

NOUN /æk'tɪnɪk_reɪ/
11 LETTERS · STARTS WITH A

actinic ray is a valid 11-letter word worth 17 points in Scrabble and 19 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
A1C3T1I1N1I1C3R1A1Y4
1 + 3 + 1 + 1 + 1 + 1 + 3 + 0 + 1 + 1 + 4 =17 PTS
DOUBLE WORD → 34TRIPLE WORD → 51
MEANING

What does actinic ray mean?