photoresponse

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. Proto-Indo-European*spend-
11. Proto-Indo-European*spondéyeti
12. Proto-Italic*spondejō
13. Proto-Italic*spondeō
14. Latinspondeō
15. Latinrespondeō
16. Latinrespōnsum
17. Old Frenchrespuns
18. Middle Englishrespounse
19. Englishresponse
20. English todayphotoresponse

From photo- + response.