How does retinal change shape when it absorbs light?

When retinal absorbs light, it undergoes a process known as photoisomerization, which changes its molecular shape while it remains attached to the protein opsin.1

In its native, unilluminated state within the rhodopsin molecule, retinal exists in a folded shape called 11-cis. Upon the absorption of a quantum of light, the molecule is converted into a straighter shape known as all-trans. This change occurs because the same group of atoms in the retinal molecule can be twisted into different configurations—a phenomenon known as cis-trans isomerism—without altering the sequence of the atoms themselves.1

This alteration in shape is the fundamental photochemical event that triggers a series of structural changes in the opsin protein, eventually leading to the electrical signals sent to the optic nerve.1