photoreactive

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Proto-Italic*wre-
9. Latinre-
10. Old Frenchre-
11. Middle Englishre-
12. Englishre-
13. Proto-Indo-European*h₂eǵ-
14. Proto-Indo-European*h₂éǵeti
15. Proto-Italic*agō
16. Latinagō
17. Latināctus
18. Latinācta
19. Old Frenchact
20. Middle Englishacte
21. Englishact
22. Englishreact
23. Englishreactive
24. English todayphotoreactive

From photo- + reactive.