How do the three opsin molecules respond to different wavelengths of light when their retinal molecular structures are identical?

The three opsin molecules respond to different wavelengths of light because of the different structure of the protein bound to the opsin molecules.

Explanation:

Opsin is a protein that is released by the action of light and forms part of the visual pigment rhodopsin. There are two groups of protein termed opsins,  type I opsins, which are employed by prokaryotes and by some algae  and fungi, and type II opsins which are used by animals.

Though the retinal molecular structures of the opsin molecules are identical, they respond to different wavelengths of light because of the different structure of the protein bound to the opsin molecules.