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# When is rotation around double bonds to form cis-trans isomers allowed?

The Rhodopsin molecules in the retain contain a derivative of vitamin A called cis-retinal (Figure 6). On exposure to light, cis-retinal becomes an isomer called trans-retinal (Textbook Question).

This process helps cause an electrical impulse to be transferred to the brain. Around which numbered bonds does the isomerism rotation occur?

The answer is bond 11 (shown in red).

What I don't understand is: I thought rotation is not allowed around double bonds.

So when light hits Figure $$6$$, how does it cause rotation around bond $$11$$ to form Figure $$7$$?