A photo pigment is a molecule in a photoreceptor cell (a rod or cone) that can trap the energy in visible light and convert it into a signal that can trigger a nerve cell.
When the nerve cell is triggered, your nervous system knows that light is hitting that particular photoreceptor. The way pigments usually work is by having a 3-dimensional structure with a special arrangement of chemical bonds.
When light of the correct wavelength
hits the pigment, the electrons in the bonds are
moved around, and this changes the 3-D structure of the pigment. When the pigment changes shape, it causes the photoreceptor cell to send a nerve impulse to the brain. The pigment molecule then has to return to its original 3-D shape to be able to be stimulated again.
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