photoactivity

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-Indo-European*h₂eǵ-
9. Proto-Indo-European*h₂éǵeti
10. Proto-Italic*agō
11. Latinagō
12. Ancient Greekἐνεργητικός (energētikós)
13. Latināctīvus
14. Latināctīvitās
15. Middle Frenchactivité
16. Englishactivity
17. English todayphotoactivity

From photo- + activity.