photoacoustic

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*h₂eḱ-
9. Proto-Indo-European*h₂ew-
10. Proto-Indo-European*h₂ṓws
11. Proto-Indo-European*h₂ḱh₂owsyéti
12. Proto-Hellenic*akóuhō
13. Ancient Greekἀκούω (akoúō)
14. Ancient Greekᾰ̓κουστῐκός (ăkoustĭkós)
15. Medieval Latinacousticus
16. Englishacoustic
17. English todayphotoacoustic

From photo- + acoustic.